U.S. patent number 8,708,560 [Application Number 12/676,890] was granted by the patent office on 2014-04-29 for method and apparatus for adjusting the color properties or the photometric properties of an led illumination device.
This patent grant is currently assigned to Arnold & Richter Cine Technik, GmbH & Co. Betriebs KG. The grantee listed for this patent is Regine Kraemer. Invention is credited to Regine Kraemer.
United States Patent |
8,708,560 |
Kraemer |
April 29, 2014 |
Method and apparatus for adjusting the color properties or the
photometric properties of an LED illumination device
Abstract
The invention relates to a method for the temperature-dependent
adjustment of the color properties or the photometric properties of
an LED illuminating device having LEDs emitting light of different
colors or wavelengths or LED color groups emitting light of the
same color or wavelength within a color group, the luminous flux
portions thereof determine the color of light, color temperature
and/or the chromaticity coordinates of the light mixture emitted by
the LED illuminating device.
Inventors: |
Kraemer; Regine (Munich,
DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Kraemer; Regine |
Munich |
N/A |
DE |
|
|
Assignee: |
Arnold & Richter Cine Technik,
GmbH & Co. Betriebs KG (Munich, DE)
|
Family
ID: |
40139949 |
Appl.
No.: |
12/676,890 |
Filed: |
September 8, 2008 |
PCT
Filed: |
September 08, 2008 |
PCT No.: |
PCT/EP2008/061887 |
371(c)(1),(2),(4) Date: |
March 05, 2010 |
PCT
Pub. No.: |
WO2009/034060 |
PCT
Pub. Date: |
March 19, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
|
US 20100301777 A1 |
Dec 2, 2010 |
|
Foreign Application Priority Data
|
|
|
|
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Sep 7, 2007 [DE] |
|
|
10 2007 044 556 |
|
Current U.S.
Class: |
374/162; 374/120;
345/84; 250/494.1; 362/612; 374/178 |
Current CPC
Class: |
H05B
45/28 (20200101); H05B 45/22 (20200101); H05B
45/20 (20200101); H05B 45/325 (20200101) |
Current International
Class: |
G01K
11/12 (20060101); G01K 7/01 (20060101) |
Field of
Search: |
;374/120,130,161,162,178,1 ;362/800,612,555,345,84 ;252/586 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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202005001540 |
|
Jun 2005 |
|
DE |
|
2004-517444 |
|
Jun 2004 |
|
JP |
|
WO02/47438 |
|
Jun 2002 |
|
WO |
|
WO 02/052901 |
|
Jul 2002 |
|
WO |
|
WO 2005/011006 |
|
Feb 2005 |
|
WO |
|
WO2006/126124 |
|
Nov 2006 |
|
WO |
|
WO2007/019663 |
|
Feb 2007 |
|
WO |
|
WO2007/090283 |
|
Aug 2007 |
|
WO |
|
Other References
English translation of International Preliminary Report on
Patentability dated Apr. 7, 2010 for corresponding International
Application No. PCT/EP2008/061887, 11 sheets. cited by applicant
.
International Search Report, dated Jan. 15, 2009, corresponding to
PCT/EP2008/061887. cited by applicant.
|
Primary Examiner: Verbitsky; Gail
Attorney, Agent or Firm: Christie, Parker & Hale,
LLP
Claims
The invention claimed is:
1. A method for the temperature-dependent adjustment of color
properties or photometric properties of an LED illuminating device
having a plurality of LEDs in LED color groups emitting light of
different colors or wavelengths, wherein luminous flux portions
thereof determine the color of light, color temperature and/or
chromaticity coordinates of a light mixture emitted by the LED
illuminating device and are adjusted by controlling the LEDs
comprising colored and white LEDs having the same color in each
case by pulse-width modulated control signals, comprising:
measuring a temperature within the LED illuminating device, of a
board containing the LEDs or a junction temperature of at least one
LED; adjusting a light mixture having a specified color of light,
color temperature and/or chromaticity coordinates by adjusting
pulse-width modulated control signals corresponding to the luminous
flux portions of the LEDs of the light mixture; determining a
dependency of the pulse-width modulated control signals on
temperature from a brightness of the LEDs varying over a relevant
temperature range by, determining a factor (f.sub.PWM)
corresponding to the reciprocal of a relative brightness
modification of the LED color groups with respect to the basic
setting, and determining a value of the pulse-width modulated
control signals (PWM(T)) of each LED corresponding to the measured
temperature by multiplying the basic-setting relating value of the
pulse-width modulating control signals (PWM.sub.A) of each LED with
the factor (f.sub.PWM) being dependent on the measured temperature
(T) according to the equation PWM(T)=PWM.sub.A*f.sub.PWM; modifying
the pulse-width modulated control signals PWM (T) corresponding to
the luminous flux portions of the LEDs of the light mixture
adjusted to a specified color of light, color temperature and/or
chromaticity coordinates, the modulation being dependent on the
measured temperature; and adjusting the pulse-width modulated
signals (PWM (T)) of each LED at the LED illuminating device by,
measuring the actual brightness of the LED illuminating device,
determining the difference between the measured brightness actual
value and a brightness set value, and adapting the light intensity
emitted from the LED illuminating device to the brightness set
value by correspondingly increasing or decreasing electric power
fed to the LEDs.
2. A method for the temperature-dependent adjustment of color
properties or photometric properties of an LED illuminating device
having a plurality of LEDs in LED color groups emitting light of
different colors or wavelengths, wherein luminous flux portions
thereof determine the color of light, color temperature and/or
chromaticity coordinates of a light mixture emitted by the LED
illuminating device and are adjusted by controlling the LEDs
comprising colored and white LEDs having the same color in each
case by pulse-width modulated control signals, comprising:
measuring a temperature (T) within a housing of the LED
illuminating device or within the area of at least one LED of the
LED color groups, determining temperature-dependent factors
f.sub.Y=f.sub.PWM for each LED color group from characteristic
lines stored for each LED color group in calibration data,
f.sub.Y=f.sub.PWM=Y.sub.0(T.sub.0)/Y.sub.0(T), with Y.sub.0=f(T);
calculating new pulse-width modulated control signals PWM(T) to
control the LEDs from the multiplication of PWM control signals
PWM(A) specified for a basic temperature (T.sub.0) to control the
LEDs with the determined temperature-dependent factors
f.sub.Y=f.sub.PWM for each LED color group,
PWM(T)=PWM(A)*f.sub.PWM; and controlling the LEDs by the new
pulse-width modulated control signals PWM(T) for each LED color
group, wherein, the pulse-width modulated control signals PWM(A)
specified for a basic temperature being determined for the
pulse-width modulated control signals for each LED color group for
light mixing ratios with specified color temperatures (CCT) or
chromaticity coordinates (x,y) as well as a brightness (Y.sub.0)
dependently on the temperature (T) of a specified temperature range
as calibration data and are stored for each LED color group as
function or table Y.sub.0=f(T) and PWM(A)=f(CCT) or
PWM(A)=f(x,y).
3. A method for the temperature-dependent adjustment of color
properties or photometric properties of an LED illuminating device
having a plurality of LEDs emitting light of different colors or
wavelengths, wherein luminous flux portions thereof determine the
color of light, color temperature and/or chromaticity coordinates
of a light mixture emitted by the LED illuminating device and are
adjusted by controlling the LEDs comprising colored and white LEDs
having the same color in each case by pulse-width modulated control
signals, comprising: measuring a temperature-dependent spectra of
the LEDs; calculating temperature-dependently optimized PWM control
signals PWM(T) for the pulse-width modulated control signals of
each LED for light mixing ratios with specified settings for color
temperature or chromaticity coordinates; storing the
temperature-dependently optimized PWM control signals PWM(T) for
the pulse-width modulated control signals of each LED for light
mixing ratios with specified settings for color temperature or
chromaticity coordinates; measuring a temperature (T) within a
housing of the LED illuminating device and/or within the area of at
least one LED; and determining actual temperature-dependent PWM
control signals PWM(T) for each LED from the stored
temperature-dependent optimized PWM control signals for the
pulse-width modulated control signals of each LED for light mixing
ratios with specified settings for color temperature or
chromaticity coordinates, and controlling the LEDs by the
temperature-dependent PWM control signals PWM(T).
4. A method for the temperature-dependent adjustment of the color
properties or the photometric properties of an LED illuminating
device having LEDs emitting light of different colors or
wavelengths, wherein luminous flux portions thereof determine the
color of light, color temperature and/or chromaticity coordinates
of a light mixture emitted by the LED illuminating device and are
adjusted by controlling the LEDs comprising of colored and white
LEDs and being grouped together to LED color groups having the same
color in each case by pulse-width modulated control signals, by
controlling the color of the LED illuminating device by a
temperature characteristic line (Y=f(Tb)) of the LED illuminating
device representing a brightness (Y) depending on a board
temperature (Tb) of the LEDs being arranged on a board and/or the
junction temperature of at least one LED for each LED color or LED
color group at a specified current in the steady state and
determining the temperature characteristic lines of the LED
illuminating device by: determining a function of the brightness
(Y) depending on the board temperature (Tb) for each LED color at a
specified current in the steady state (Y=f(Tb)); normalizing the
characteristic lines onto (Y(Tb1)=1), wherein (Tb1) is an
arbitrarily chosen temperature value close to the later working
point; determining the parameters (a, b, c, d) for a linear
function having the form Y(Tb)=a+b*Tb, a second-degree polynomial
having the form Y(Tb)=a+b*Tb+c*Tb.sup.2, or a third-degree
polynomial having the form (Tb)=a+b*Tb+c*Tb.sup.2+d*Tb.sup.3; and
storing the parameters (a, b, c, d) in illuminating modules of the
LED illuminating device, in the LED illuminating device or in an
external controller.
5. The method of claim 4, wherein an LED illuminating device is
comprised of a plurality of LED modules the temperature
characteristic lines of which for a temperature-dependent
adjustment of color properties or photometric properties of the LED
illuminating device, having LEDs emitting light of different colors
or wavelengths or LED color groups emitting light of the same color
or wavelength within a color group, wherein luminous flux portions
thereof determine the color of light, color temperature and/or
chromaticity coordinates of the light mixture emitted by the LED
illuminating device, is determined by: determining temperature
characteristic lines randomly; converting characteristic line
parameters onto individual dominant wavelengths by means of
interpolation or extrapolation; transferring the determined
characteristic lines onto all LED modules; and storing the
determined characteristic lines in the memory of said LED modules.
Description
CROSS-REFERENCE TO A RELATED APPLICATION
This application is a National Phase Patent Application of
International Patent Application Number PCT/EP2008/061887, filed on
Sep. 8, 2008, which claims priority of German Patent Application
Number 10 2007 044 556.5, filed on Sep. 7, 2007.
BACKGROUND
The invention relates to a method for adjusting the color
properties or photometric properties of an LED spotlight as well as
an apparatus.
Illuminating spotlights having light emitting diodes (LEDs) are
known which are used, e.g., as camera attachment light for film and
video cameras. Since the LEDs used therefore have either the color
temperature "daylight white" or "warm white", a continuous or exact
activation or switch from a warm white to a daylight white color
temperature having defined standard color value portions close to
or on the Planckian locus is not possible and the color
reproduction at film and video recordings is unsatisfactory.
Typical film materials for film recordings like "cinema color
negative film" are optimized towards daylight having a color
temperature of 5600 K or for incandescent light having a color
temperature of 3200 K and achieve extraordinary color reproduction
properties for illuminating a set with those light sources. If
other artificial light sources are used during film recordings for
illuminating a set, they have to be adjusted on the one hand to the
optimum color temperature of 3200 K or 5600 K and on the other hand
have to have very good color reproduction quality. Regularly, for
this purpose the best color reproduction grade having a color
rendering index of CRI .gtoreq.90 . . . 100 is required.
For an LED spotlight consisting of more than three LED colors,
there are unlimited possibilities or possibilities only limited by
the resolution of the controlling to adjust a desired chromaticity
coordinate color like e.g., x/y=0.423/0.399, CCT=3200 K by mixing
the used primary colors. Depending on the mixing ratio, it can be
optimized towards different parameters like luminous efficacy or
color reproduction. In case of a spotlight primarily used for film
and TV recordings, the mixture can additionally be optimized
towards the color reproductions properties of the film material or
of the sensor of a digital camera. If this optimization is not
done, in the most unlikely event the correct chromaticity
coordinates x/y are adjusted, but having very unfavorable color
reproduction properties. In particular, due to the narrow band
spectra of the LED colors like blue, green, red, spectra easily
result having an inacceptable color reproduction. Or, however,
spectra having good to very good color rendering indices
(CRI.gtoreq.90) which generate at recordings with film or digital
cameras significant color deviations as compared to usual light
sources like tungsten incandescent or daylight.
It can be deduced from the colorimetry that for such total spectra
generated from narrowband LED spectra, optionally also in
combination with luminescent material LEDs, never all colorimetric
values (chromaticity coordinates, color rendering index as well as
mixed-light capability) being relevant for the film and video
illumination can adopt ideal values at the same time. Nonetheless,
very good results can be achieved if it is guaranteed that none of
the optimization parameters deviates too far from the ideal value.
However, in the colorimetry no general algorithm is known as to in
which ratio more than three spectra have to be mixed to achieve
values being as good as possible for the desired chromaticity
coordinate, color rendering index as well as mixed-light capability
with film at the same time.
However, as in the case of using fluorescent tubes for the
illumination of film or video recordings, it can occur in case of
artificial light sources having a none-continuous spectral power
distribution that these light sources achieve the required values
for the color temperature and color rendering index, but
nonetheless have a significant color deviation in case of using
them for film recordings as compared to tungsten incandescent or
HMI lamps or daylight. In this case, one speaks about an
insufficient mixed-light capability. This effect can also occur in
case of using variously colored LEDs in an LED spotlight. During a
test with an LED combination optimized towards a color temperature
of 5600K and a color rendering index of CRI=96 at film recordings,
a massive red cast as compared to HMI lamps was observed. Also
tries with daylight white LEDs did not result in satisfactory
results with respect to the mixed-light capability.
US 2004/0105261 A1 discloses a method and an apparatus for emitting
and modulating light having a specified light spectrum. The known
photometric device has several groups of light emitting
apparatuses, each group of which emits a specified light spectrum,
and a control device controls the energy supply to the single light
emitting apparatuses in such a way that the overall resulting
radiation has the specified light spectrum. Thereby, by combination
of daylight white and warm white LEDs and modifications of the
intensities any color temperatures between the warm white and the
daylight white LEDs can be adjusted.
A disadvantage of this method is the also not optimal color
reproduction in case of film or video recordings and the lacking
possibility to adjust a specified color temperature and an exact
chromaticity coordinate. Dependent on the choice of the individual
LEDs or the groups of LEDs and the respectively adjusted color
temperature, one faces thereby partially significant color
deviations from the Planckian locus which can only be corrected by
using corrections filters. Additionally, the luminous efficacy is
not optimal in case of a warm white setting of the combination of
daylight white and warm white LEDs, since hereby relatively high
converting losses occur due to the secondary emission of the
luminescent material. A further disadvantage of this method is that
for adjusting a warm white or daylight white color temperature a
main part of the LEDs of the respective other color temperature
cannot be used or can only be used highly dimmed so that the
utilization factor for the color temperatures around 3200 K or 5600
K typically required in case of film recordings is only
approximately 50%.
From DE 20 2005 001 540 U1 a light source for daylight is known
which can be adjusted in its color temperature and by which at
least one LED emitting white light of a certain color temperature
is combined with variously colored light emitting LEDs, in
particular in the primary colors red, green and blue. By a
modification of the power of single LED colors, a certain color
temperature or certain standard light quality can be adjusted by
tuning or correcting a specified color temperature or standard
light quality automatically by the use of suited sensors, logic and
software which can detect the actual spectral power distribution of
the light source.
By the use of variously colored LEDs in illuminating spotlights, in
particular for photographic or cinematographic recordings, the
light of which has a specified color temperature and color
rendering index and owns a sufficient mixed-light capability, the
following problems occur.
Since LEDs do not emit the emitted light in a monochromatic way
with a sharp spectral line but with a band spectrum having certain
width so that the emission spectrum of an LED can be assumed as
Gaussian bell-shaped curve or as sum of several Gaussian
bell-shaped curves and the emission spectra of LEDs can be
simulated via the Gaussian distribution. In FIG. 4 some emission
spectra of LEDs are exemplarily depicted as function of the
relative illumination density over the wavelength, from which can
be seen that the wavelength of variously colored light emitting
LEDs increases from blue light by green light, amber-colored light
towards red light and the form of the emission spectrum of white
light emitting LEDs strongly differs from the emission spectra of
LEDs emitting differently colored light. This deviation results
from the technology of white light generation which is based on the
basis of a semiconductor element emitting blue light an being
provided with a phosphor covering converting the blue light
partially into yellow light resulting in a second, peak in the
yellow area of the spectrum besides the first peak in the
wavelength area of blue light, a mixed result of which are the
portions of white light. Thereby, via the thickness of the phosphor
covering, the color temperature can be varied so that in this
manner yellowish, warm white as well as daylight white LEDs can be
produced.
Additionally, LEDs as illuminant have a strong temperature
dependency. With increasing junction temperature, the properties
and characteristics of LEDs vary significantly, wherein with
increasing temperature the luminance decreases strongly. This is
based on the fact that at higher temperature the portion of the
radiation-free recombination increases and with increasing
temperature a shift of the emission spectra towards higher
wavelengths, i.e., towards the red spectrum, is effected. FIG. 5
shows in a schematic depiction the relative luminance over the
junction temperature of LEDs which emit blue, green and red light
and consist of different material combinations. As a result, the
temperature dependency of LEDs is differently strong pronounced in
dependence on the used materials what results in the fact that also
the colorimetric properties of a light mixture being additively put
together from variously colored LEDs vary to achieve a certain
color of light or color temperature.
To achieve the color tint or the color temperature of an
originally, e.g. at an initial temperature of 20.degree. C.,
adjusted basic mixture of the light emitted from variously colored
LEDs also at a temperature differing from the initial temperature,
a spectrometer can be provided and, e.g., be used in the area of
the front lens of an illuminating spotlight, which spectrometer
measures the spectrum of the light emitted from the illuminating
spotlight, or a color sensor is used in the area of the light
emitting plane, which color sensor registers deviations of the
actual color of the spotlight and then detects the intensity as
well as the chromaticity coordinates of the LEDs participating in
the light generation in a pulse/measuring mode. Thus, shifts of the
peak wavelength as well as variations of the height of the peak
wavelength can be detected and, as actual values term, can be fed
to a regulation device, the set value of which is the basic setting
or basic mixture of the light emitted from the illuminating
spotlight. By an according comparison between the set value and the
actual value, the light mixture can be corrected to maintain the
original spectrum of the basic mixture.
Such a regulation of the color temperature of the light being
emitted from an LED spotlight is very complex and time-consuming
due to the necessary use of an expensive color sensor and its
arrangement in the optical path of the LED spotlight as well as due
to the necessary use of a suited computer in connection to a
regulation device since in case of such a regulation a
temperature-dependent variation of the peak wave length of all LED
colors used in the LED spotlight has to be detected and has to be
considered during the regulation. The time necessary for this is,
e.g., in case of film recordings under different ambient conditions
not always available.
SUMMARY
It is an object of the instant invention to adjust and keep
constant the color of light, color temperature or the chromaticity
coordinates of a light mixture emitted from an LED spotlight with
minimal cost and time effort independently from the ambient
temperature of the LED spotlight.
The solutions according to the invention guarantee an adjustment of
and a compliance with the color of light, color temperature or the
chromaticity coordinates of a light mixture being emitted from an
LED spotlight and being composed of luminous flux portions of
variously colored LEDs independently on the temperature, in
particular on the board temperature of the LEDs, under a minimum
production and time effort.
The method according to the invention starts from different
approaches and enables different adjustment accuracies with the
different production and time effort for achieving an adjustment of
the color of light, color temperature or the chromaticity
coordinate of the light mixture independently on the ambient
temperature of the LED spotlight. The production effort and the
control or regulation time for the compliance of the desired color
of light, color temperature or the chromaticity coordinate of the
light mixture being emitted from the LED spotlight is overall
significantly smaller than the production and regulation time
effort when using a plurality of color sensors since in case of the
method according to the invention only one temperature sensor is
necessary as actual value indicator for a compliance of the color
of light, the color temperature or the chromaticity coordinates of
the light mixture being emitted from the LED spotlight and the
regulation time is only minimal dependent on the used method in
each case.
A first alternative method for the color stabilization of an LED
spotlight at different ambient temperature is characterized by a
basic setting of the light mixture onto a specified color of light
by an adjustment of the luminous flux portions of the variously
colored LEDs at an initial temperature of the LED spotlight,
determining the initial emission spectra E.sub.A(.lamda.) of the
variously colored LEDs at the basic setting, the initial emission
spectra being dependent on the wavelength of the variously colored
LEDs, determining the emission spectra E(.lamda.) depending on the
wavelength of the variously colored LEDs at a measured temperature
of the LED spotlight differing from the initial temperature,
determining the luminous flux portions of the variously colored
LEDs for a light mixture having the specified color of light at the
measured temperature, adjusting the determined luminous flux
portions of the variously colored LEDs at the LED spotlight.
In case of this first method according to the invention firstly a
calibration of the spotlight is effected with an optimum adjustment
of the luminous flux portions of variously colored LED color groups
for a desired color of light of the light mixture emitted from the
LED spotlight in a basic setting of the LED spotlight. During a
variation of the ambient temperature, a temperature-dependent new
calibration for correcting the luminous flux portions of the
variously colored LEDs of the light mixture is carried out by a new
calculation of the luminous flux portions with the
temperature-dependent emission spectra of the variously colored
LEDs and an according adjustment of the luminous flux portions at
the spotlight. For this method, the emission spectra of the single
color groups of the variously colored LEDs at the measured, actual
temperature are necessary for each correction procedure, which
emission spectra have to be measured with the spectrometer--this
being, however, comparatively time consuming--so that this method
is, e.g., only limitedly applicable for film recordings, the more
so as the installation of the spectrometer in an LED spotlight is
connected to a significant production and cost effort.
Accordingly, in further developments of this solution according to
the invention, the emission spectra of the variously colored LEDs
are approximated for the measured temperature in each case by the
Gaussian distribution or by a temperature-dependent normalization
of the emission spectra determined by the calibration, this being
done in the context of a calibration as well as the thereupon-based
new calculation of the luminous flux portions dependent on the
temperature. The result, namely the luminous flux portions of the
LED colors depending on the temperature, is preferably stored in
table or function form in the spotlight since then in the spotlight
no spectra are necessary for measuring, approximation and
calculation.
Both further-developed solutions are based on the finding that the
luminance and peak wavelength as well as the half-width, i.e., the
width of the emission spectrum at 50% of the relative luminance of
the peak wavelength of the emission spectra are dependent on the
measured temperature in a linear or quadratic (luminance of yellow,
amber, red) way. By those methods, the spectra for all color groups
of the variously colored LEDs can be newly calculated from the
temperature measured in each case.
The approximation of the emission spectra of the variously colored
LEDs by the Gaussian distribution is based on the fact that the
emission spectra of LEDs can be simulated with the aid of the
Gaussian bell-shaped curve
.function..lamda.e.lamda..lamda. ##EQU00001## sufficiently precise
by determining the peak wavelength .lamda..sub.p of the LED
emission spectrum and the half-width w.sub.50 of the LED emission
spectrum, the peak wavelength and the half-width being linearly
dependent on the temperature for each group of same-color LEDs. The
temperature-dependent intensity factor fL serves for adjusting the
intensity of the simulated spectrum onto the intensity of the
spectrum at a determined ambient temperature. The function of the
intensity of the spectrum depending on the temperature is for each
LED color a linear or quadratic function. Thus, if the parameters
.lamda..sub.p and w.sub.50 being linearly dependent on the
temperature are known from the basic setting of the light mixture
of the LED spotlight during its calibration as well as the
temperature-dependent factor fL or the linear or quadratic function
of the intensity depending on the temperature, then the respective
relative emission spectrum of the single color groups of the
variously colored LEDs can be suggested at temperatures differing
from the initial temperature so that deviations of the emission
spectra from the basic setting can be determined and
compensated.
Based on the Gaussian distribution, the emission spectrum of the
variously colored LEDs and therewith of the light mixture of the
light emitted from the LED spotlight can be approximated even more
precise if the emission spectra E(.lamda.) depending on the
wavelength of the variously colored LEDs are simulated according to
the formula
.function..lamda..times..pi.e.times..lamda..lamda. ##EQU00002## by
determining the peak wavelength .lamda..sub.p of the LED emission
spectrum, the half-width w.sub.50 of the LED emission spectrum and
a temperature-dependent intensity factor f.sub.L, the peak
wavelength and the half-width being linearly dependent on the
temperature for each group of same-color LEDs.
The parameters peak wavelength .lamda..sub.p and half-width
w.sub.50 used in this approximation formula are for all color
groups of the variously colored LEDs linearly or quadratically
dependent on the temperature. The temperature-dependent conversion
factor f.sub.L(T) thereby represents a normalization factor which
refers the approximated spectrum to the measured relative luminance
dependent on the temperature. The measured dependency of a maximum
spectral radiant power on the temperature can also be used as
substitute for the factor fL(T). Thus, all necessary parameters can
be determined and the emission spectra can be calculated from a
measured temperature value. In this manner, e.g., an approximation
of the emission spectra for the color groups amber, blue, green and
red is possible.
The determination of the emission spectrum for white LEDs thereby
represents a special case since in case of an LED emitting white
light a blue LED having a phosphor covering is concerned so that
the emission spectrum shows two peaks, namely one peak in the blue
and one peak in the yellow spectral area. Thereby, a simple
approximation by a Gaussian distribution is not possible, however,
both peaks can be approximated by a Gaussian distribution in each
case.
In an embodiment of the method according to the invention, the
emission spectrum for white LEDs is accordingly approximated by
several Gaussian distributions, preferably by three or four
Gaussian distributions. Thereby, a third Gaussian distribution is
subtracted from the two Gaussian distributions determining the two
peaks in the emission spectrum in order to approximate the
calculated spectrum within the "valley" at about 495 nm lying
between the two peaks towards the measured emission distribution.
An even more precise approximation of the calculated emission
spectrum towards a measured emission distribution can be achieved
by adding a fourth Gaussian distribution, however, an approximation
by three Gaussian functions turns out as sufficient compromise
between maximum accuracy and minimum calculation effort.
The methods according to the invention for the approximation of the
emission spectra of the variously colored LEDs for a generation of
the desired light mixture of the LED spotlight have the advantage
of a sufficiently precise approximation of the calculated emission
spectra to actually measured emission spectra, wherein the shift of
the peak wavelength and modifications of the half-width are
accounted for so that the light mixture being composed of the light
of variously colored LEDs can be corrected very precisely.
Comparative measurements have shown that the color temperature
after this correction amounts to 28 K for artificial light or
tungsten and 125 K for daylight at visibility thresholds of 50 K
for tungsten or 200 K for daylight, whereas without color
correction the shift amounts to 326 K for tungsten and 780 K for
daylight and lies therewith in the clearly visible area.
A disadvantage of this approximation of the emission spectra
dependent on the ambient temperature of the LED spotlight exists in
the fact that for the calculation of the single color groups of the
variously colored LEDs three temperature-dependent parameters in
each case and for the special case of the white color nine
temperature-dependent parameters and therewith altogether 21
temperature-parameters have to be calculated for the calculation of
the actual emission spectrum for a correction of the system for a
compliance with the desired color of light or color temperature of
the light mixture adjusted at an initial temperature. This means a
significant effort as compared to the subsequently described
alternative method for the approximation of the emission spectra of
an actual temperature by a temperature-dependent
shift+normalization of the calibration of the emission spectra
determined at an initial temperature.
In case of this alternative method ("shift of peak wavelength") the
emission spectra E(.lamda.) being dependent on the wavelength of
the variously colored LEDs are approximated at a measured
temperature of the LED spotlight differing from the initial
temperature by a temperature-dependent shift and normalization of
the initial emission spectra E.sub.A according to
E.sub.T(.lamda.)=f.sub.L(T)f.sub.VL(T)E.sub.A(.lamda.-.DELTA..lamda..sub.-
p(T)) wherein f.sub.L (T) represents a temperature-dependent
conversion factor (measured luminance of the spectrum relative to
the luminance of the initial spectrum) representing a relative
luminance decrease over the whole temperature range,
.DELTA..lamda..sub.p(T) denotes a shift of the peak wavelength as
compared to the initial spectrum depending on the temperature and
f.sub.VL(T) represents a normalization factor which normalizes the
spectrum shifted by .DELTA..lamda..sub.p (T) onto the same
luminance like that of the original spectrum (necessary due to the
other position with respect to the V(.lamda.) curve).
In case of this alternative method, the emission spectra are
shifted by the modification of the peak wavelength in the basic
setting of the LED spotlight which is recorded during the
calibration of the LED spotlight, afterwards they are normalized
with the factor f.sub.VL (T) again onto the initial luminance of
the spectra and are finally considered with a temperature-dependent
factor. The factor f.sub.L (T) represents the measured relative
luminance decrease over the whole temperature range so that the
emission spectra multiplied with factors f.sub.L (T)f.sub.VL (T) of
the shifted initial mixtures are adjusted with respect to the
luminance onto the actual emission spectra at the actual
temperature in each case. To account for shifts of the peak wave
length of the single color groups of the variously colored LEDs,
the emission spectra are shifted along the abscissa indicating the
wavelength in case of a depiction of the relative luminance over
the wavelength.
The advantage of this method for the approximation of the emission
spectra at various ambient temperatures of the LED spotlight exists
in the fact but in opposite to the approximation of the emission
spectra by the Gaussian distribution that only 10 simple to be
determined instead of 21 temperature-dependent parameters have to
be calculated which results in a significantly reduced calculation
effort and a smaller susceptibility to errors. Disadvantageous as
compared to the approximation of the emission spectra by the
Gaussian distribution is, however, that the shift of peak
wavelength is less precise since the modification of the half-width
as well as the shoulder distribution of the emission spectra is not
considered.
In case of both precedingly described methods for the approximation
of the emission spectra of the variously colored LEDs for the color
stabilization of an LED spotlight, the emission spectra at an
ambient temperature of the LED spotlight different from the initial
temperature in the basic setting, these emission spectra differing
from the emission spectra of the variously colored LEDs in the
basic setting during the calibration of the LED spotlight, are
converted into a modification of the luminous flux portions of the
respective color groups of the variously colored LEDs for the
correction of the light mixture. Therefore and for the use of a
further, subsequently described method for the determination of the
emission spectra of variously colored LEDs at an ambient
temperature of the LED spotlight differing from the initial
temperature, a program-controlled processing unit is used into
which the determined emission spectra of the used LED colors or the
emission spectra of desired LED colors are put in, several
optimization parameters are adjusted and from which luminous flux
portions optimized towards different target parameters for the
variously colored LEDs are determined or are provided to an
electronics controlling the variously colored LEDs.
The program-controlled processing unit serves for calculation of
light mixtures on the basis of variously colored LEDs by making it
possible with the aid of the emission spectra of the variously
colored LEDs both to determine the color properties of light
mixtures of the light sources having various luminous flux portions
and to calculate optimized light mixtures for certain kinds of
light. Thereby, up to five emission spectra can be chosen, imported
and the best possible mixture for specified color properties can be
calculated via an optimization function. Further, different kinds
of light used in the film production, as, e.g., tungsten
incandescent light 3200 K for artificial light or tungsten and
daylight or HMI light 5600 K for daylight can be chosen, wherein
via further options by the input of optimization and target
parameters the pre-settings can be fine-tuned to achieve an optimum
light mixture. Additionally, the program-controlled processing unit
offers the possibility to determine the colorimetric properties of
a manually adjusted mixture so that it is, e.g., possible to
examine the modifications of mixtures having the same portions but
different emission spectra.
The desired color temperature of the light mixture produced by the
variously colored LEDs, the mixed-light capability and the
reference illuminant as well as the film material or the camera
sensor for which a good mixed-light capability is to be achieved
are adjustable as optimization parameters, whereas the target
parameters for the optimization of the luminous flux portions
consist of one or several of the parameters color temperature,
minimum distance from the Planckian locus, color rendering index
and mixed-light capability with film or digital camera and set
values and/or tolerance values can be entered for the target
parameters.
The LED spotlight can be adjusted with the luminous flux portions
determined by the program-controlled processing unit for the
temperature-dependent color correction onto the newly calculated
light mixture in each case. The calculation can also be effected
online within the spotlight or in advance in the context of the
calibration and the determined results (luminous flux portions of
the LED colors depending on the temperature) can be stored in table
form or as a function in the internal memory of the spotlight. To
correct possible deviations of the luminance which can occur after
the correction, a luminance measurement with a V(.lamda.) sensor is
additionally effected according to a further feature of the
solution according to the invention so that the LED spotlight is
adapted to the luminance set value from the difference between the
actual luminance and the set value of the luminance via a
corresponding increase or decrease of the electric power fed to the
variously colored LEDs.
Since the spectral distribution of the emission of the variously
colored LEDs very strongly depends on the current intensity, and in
case of LED types in the blue and green area the dominant
wavelength decreases with increasing current intensity, whereas in
case of the LED types amber and red the dominant wavelength
increases with increasing current intensity, a shift of the
dominant wavelength of several nanometers would occur in a light
mixture, i.e., an additive composition of the light emitted from an
illuminating spotlight and made of the light emitted from the color
groups of variously colored LEDs in case of a partial control by
the current intensity of the variously colored LEDs to achieve a
desired light mixture so that the color temperature of the light
mixture emitted from the illuminating spotlight would significantly
change.
Due to the strong dependency on the current of the LEDs, a partial
control of the LEDs and therewith of the light mixture is not a
effected via a regulation of the current intensity but via a
pulse-width modulation having essentially rectangular-shaped
current impulses of adjustable pulse-width and impulse pauses lying
there between which form together a periodic time of the
pulse-width modulation. A partial control or dimming is thereby
effected by a variation of the pulse-width of the rectangular
signal at a fixed basic frequency so that the rectangular impulse
has the half width of the whole period in case of a 50%
dimming.
Generally, one could of course also carry out an analogous dimming
despite the above-described effect of the shift of the dominant
wavelength dependent on the current if this shift is accordingly
accounted or compensated for during the determination of the
luminous flux portions. Only for the sake of simplicity, an
operation with pulse-width modulation (PWM) is preferred. The
operation frequency is preferably >20 kHz to avoid beats at high
speed film recordings.
Accordingly, a further feature of the solution according to the
invention exists in the fact that the luminous flux portions of the
variously colored LEDs are controlled by controlling the variously
colored LEDs by pulse-width modulation. This control is effected in
connection to the previously explained emission of the luminous
flux portions for the variously colored LEDs from the
program-controlled processing unit by providing pulse-width
modulated signal portions corresponding to the luminous flux
portions to an electronics controlling the variously colored
LEDs.
Thereby, a color stabilization of an LED spotlight is ensured by
which--independently on a varying ambient temperature of the LED
spotlight--the color of light or color temperature or the
chromaticity coordinates of a desired light mixture as well as
optionally further parameters which influence the light emitted
from the LED spotlight like the color rendering index or the
mixed-light capability, the luminous flux portions of the color
groups of the variously colored LEDs are tracked or corrected.
Since for tracking the luminous flux portions at different ambient
temperatures only one temperature sensor is necessary and all
parameters being necessary for the determination of the respective
emission spectra of the variously colored LEDs can be pre-entered,
the precedingly described methods for the determination of the
emission spectra enable in connection to the program-controlled
processing unit and a control electronics providing pulse-width
modulated signals the immediate control of the single color groups
of the variously colored LEDs without the necessity of an
additional input of the user, after he or she has fixed the
optimization and target parameters in the basic setting or
calibration of the LED spotlight.
Hence, during the application of the method for the approximation
of the emission spectra of the variously colored LEDs with the aid
of the Gaussian distribution for the correction of the color
properties or photometric properties of the LED spotlight depending
on the ambient temperature the following method steps result:
measuring the temperature values at an LED of each color group of
the variously colored LEDs, determining the parameters
.lamda..sub.p, w.sub.50 and f.sub.L for each color group via a
linear or quadratic dependency on the temperature, calculating the
new, temperature-dependent emission spectra by the Gaussian
distribution with the aid of the temperature-dependent parameters,
importing the emission spectra into the program-controlled
processing unit and calculating the pulse-width modulated signal
portions corresponding to the luminous flux portions for the light
mixture, adjusting the pulse-width modulated signal portions for
the variously colored LEDs at the LED spotlight and optionally
measuring the luminance and adapting the light intensity emitted
from the LED spotlight to the luminance set value by a
corresponding increase or decrease of the electric power fed to the
variously colored LEDs.
If the preceding method steps 1 to 4 are carried out in the context
of the calibration, then the temperature-dependent luminous flux
portions can be stored in the spotlight, this being generally
faster and making more sense.
Thus, for the application of a method for the approximation of the
emission spectra of the variously colored LEDs via a
temperature-dependent shift plus normalization of the initial
spectra determined during the calibration during the basic setting
of the LED spotlight for the correction of the color properties or
photometric properties of the LED spotlight depending on the
ambient temperature, preferably the following method steps serve:
measuring the temperature values at an LED of each color group of
the variously colored LEDs, determining the parameters f.sub.L and
.DELTA..lamda..sub.p for each color group via a linear or quadratic
dependency on the temperature, calculating the new,
temperature-dependent emission spectra E.sub.T(.lamda.), importing
the temperature-dependent emission spectra E.sub.T(.lamda.) into
the program-controlled processing unit and calculating the
pulse-width modulated signal portions corresponding to the luminous
flux portions for the light mixture, adjusting the pulse-width
modulated signal portions for the variously colored LEDs at the LED
spotlight, optionally measuring the luminance and adapting the
light intensity emitted from the LED spotlight to the luminance set
value by a corresponding increase or decrease of the electric power
fed to the variously colored LEDs.
Also in case of this method, the preceding method steps 1 to 4 can
be carried out in the context of the calibration and the
temperature-dependent luminous flux portions can be stored in the
spotlight.
In both precedingly described methods, the integration of the
program-controlled processing unit for the calculation of the
luminous flux portions of the light mixture of the LED spotlight at
different ambient temperatures is necessary and offers the
advantage of a very precise calculation of the luminous flux
portions of the single color groups. In particular, in case of a
precise adjustment of the different options offered from the
program of the program-controlled processing unit for a precise
calculation of the luminous flux portions of the light mixture
non-negligible calculation times have to be considered what is not
acceptable for some application cases, e.g. at a film set since the
LED spotlight has to be available without interruptions.
As a further alternative method, there exists the possibility that
the spectra are not approximated dependent on the temperature but
are measured in the context of the calibration with very precise
data. In the context of the calibration, a new calculation of the
mixing portions depending on the temperature can be performed and
the temperature-dependent mixing portions can be stored in the
spotlight in table or in function form.
Accordingly, an alternative method for the adjustment of the color
properties or photometric properties of an LED spotlight being
composed of variously colored LEDs the luminous flux portions of
which determine the color of light, color temperature and/or the
chromaticity coordinates of the light mixture emitted from the LED
spotlight and are adjusted by controlling the variously colored
LEDs by pulse-width modulated signals, depending on the ambient
temperature of the LED spotlight exists in that the pulse-width
modulating signals controlling the variously colored LEDs
corresponding to the luminous flux portions of the single color
groups for the basic setting of the light mixture are
temperature-dependently modified to a specified color of light.
This alternative method represents a very simple solution for a
color correction at different ambient temperatures and is based on
the temperature dependency of the pulse-width modulating signals
controlling the variously colored LEDs, having the target to keep
the relative luminous flux portions of the colors participating in
the color mixture constant over the whole ambient temperature
range. By an increase or decrease of the pulse-width modulated
signal portions, the spectra emitted by an actually detected
ambient temperature are adapted to the luminous flux portions of
the initial spectra detected in the basic setting during the
calibration of the LED spotlight so that the specified light
mixture can be further used.
Thereby, the temperature dependency of the pulse-width modulated
signal portions can be determined from the modification of the
luminance. Examinations have shown that the variously colored LEDs
are indeed very differently strong temperature-dependent (LEDs
which emit in the long wave range of the visible spectrum decrease
in the luminance with increasing temperature significantly stronger
than LEDs of the short wave range), this temperature dependency of
the luminance over a big temperature range, which is important for
the practical application, can, however, be determined and
described for each color via a linear or quadratic function.
If one determines accordingly the relative luminance modification
with respect to the light mixture adjusted in the basic setting,
then one obtains a factor f.sub.PWM for each color group of the
variously colored LEDs. If the corresponding portion of the
pulse-width modulated signal for the respective LED color of the
basic setting of the light mixture is multiplied with the
reciprocal of the factor f.sub.PWM, then the new portion of the
pulse-width modulated signal for the respective LED color at the
actual measured ambient temperature is achieved out of it.
A further development of this simplified alternative method for the
color stabilization of an LED spotlight therewith exists in that a
factor f.sub.PWM corresponding to the relative luminance
modification of each color group of the variously colored LEDs with
respect to the basic setting is determined and in that the
multiplication of the value corresponding to the basic setting of
the pulse-width modulated signals PWM.sub.A of each color group
results with the reciprocal 1/f.sub.PWM of this factor being
dependent on the measured temperature results in the value of the
pulse-width modulated signals PWM (T) of each color group
corresponding to the measured temperature T according to the
formula: PWM(T)=PWM.sub.A/f.sub.PWM(T) Also in this simplified
method, possible deviations in the luminance which can occur after
determining the luminous flux portions of the variously colored
LEDs at the actual measured temperature can be corrected in that a
luminance measurement is performed with an V(.lamda.) sensor, the
difference between the measured luminance actual value and a
luminance set value is determined and the luminance emitted from
the LED spotlight is adapted by a corresponding increase or
decrease of the electric power fed to the variously colored LEDs to
the luminance set value.
An essential advantage of this correction with respect to the
normalization of the pulse-width modulated signal portions for
controlling the variously colored LEDs exists in the simplicity of
the determination of the correction factors since for a new
adjustment of the light mixture only five parameters have to be
calculated by a simple function and subsequently the original
portions have to be evaluated with these parameters. Thereby, a
calculation via a program-controlled processing unit is not
necessary so that the big portion of the calculation and
programming effort of both previously described methods for the
approximation of the emission spectra of the variously colored LEDs
and the correction of the luminous flux portions of the variously
colored LEDs is omitted.
Due to the very short calculation time, the correction for the
color stabilization of the LED spotlight can continuously take
place so that during operation of the LED spotlight stable color
properties like color temperature, color reproduction, distance
from the Planckian locus and mixed-light capability are guaranteed.
Despite the simplicity of this correction method the differences
occurring in the color values after the correction are comparably
to the precedingly mentioned color deviations by Gaussian
approximation such small that they can be neglected.
Although during the application of the different methods according
to the invention for the color stabilization of an LED spotlight at
different ambient temperatures to guarantee a low production and
time effort no color sensors are necessary, but only a temperature
sensor is needed, for, e.g., considering aging processes the output
signals of a color sensor or a spectrometer additionally installed
at the LED spotlight can be accounted for during the determination
of the luminous flux portions of the color groups of the variously
colored LEDs of the light mixture in the basic setting, wherein the
output signals of the color sensor or the spectrometer are provided
to the program-controlled processing unit for the determination of
the luminous flux portions or the pulse-width modulated signals
corresponding to the luminous flux portions of the color groups of
the variously colored LEDs of the light mixture in the basic
setting.
If the color sensor is calibrated, on the one hand the chromaticity
coordinates x, y and the dominant wavelength of the color
calculated out of it and on the other hand the brightness of the
single LEDs can be extracted from the RGB or XYZ signals of the
color sensor. Simultaneously to the color values, the actual
temperature is read from the temperature sensor to correlate the
new measured values with the temperature-dependent characteristic
lines (.lamda.p, w50 and brightnesses) stored in the memory. From
this, the parameters intensity as well as peak wavelength being
necessary for the Gaussian approximation can be determined, the
half-width is considered as approximately constant with respect to
the original spectrum.
In the context of the color control of the LED illuminating device
a temperature-dependent power limiting is performed since the total
power of the LED illuminating device or the total current fed to
all LEDs of the LED colors must not exceed a specified, preferably
temperature-dependent threshold; because it makes less sense to
feed more current with increasing temperature and consequently
decreasing brightness of the LED illuminating device in the
expectation to therewith compensate the decrease in brightness of
single or several colors. With an increase of the current feed and
therewith of the total power of the LED illuminating device the
temperature further increases so that the luminous efficacy further
decreases, until single or several LEDs are overloaded and are
therewith destroyed or a hardware-based current limitation
intervenes.
Accordingly, a limitation of the power consumption of the LED
spotlight and/or of the total current fed to the LED is provided,
wherein the power consumption of the LED spotlight and/or of the
total current fed to the LEDs can be temperature-dependently
limited.
A further method for the temperature-dependent adjustment of the
color properties or photometric properties of an LED illuminating
device having LEDs emitting light of different color or wavelength,
the luminous flux portions of which determine the color of light,
color temperature and/or a chromaticity coordinates of the light
mixture emitted from the LED illuminating device and are adjusted
by controlling the variously colored LEDs being grouped together to
LED color groups having the same color in each case and consisting
of colored and white LEDs by pulse-width modulated control signals
is characterized by a color control of the LED illuminating device
by a temperature characteristic line (Y=f (Tb)) of the LED
illuminating device, the temperature characteristic line reflecting
the brightness (Y) depending on the board temperature (Tb) of the
LEDs arranged on a board and/or of the junction temperature of at
least one LED for each LED color or LED color group at a specified
current in the steady state.
In this method, the determination of temperature characteristic
lines of the LED illuminating device is carried out by a
determination of the function of the brightness (Y) depending on
the board temperature Tb for each LED color at a specified current
in the steady state (Y=f (Tb)), a normalization of the
characteristic lines onto (Y (Tb1)=1), wherein (Tb1) is an
arbitrarily chosen temperature value close to the later working
point, a determination of the parameters (a, b, c, d) for a linear
function of the form Y(Tb)=a+b*Tb a second-degree polynomial of the
form Y(Tb)=a+b*Tb+c*Tb.sup.2 or a third-degree polynomial of the
form Y(Tb)=a+b*Tb+c*Tb.sup.2+d*Tb.sup.3 and storing the parameters
a, b, c, d in illuminating modules of the LED illuminating device,
in the LED illuminating device or in an external controller.
For a preferably random determination of calibration correction
factors for the LED illuminating device a measurement of the
brightness (Y) and the board temperature (Tb) for each LED color is
effected immediately after turning on the LED illuminating device,
having the results Y(Tbcal, t0), measurement of the brightness (Y)
and board temperature (Tb) for each LED color in the steady state
and conversion of the brightness (Y(Tb), t1) to a board temperature
(Tb1) via the characteristic line (Y=f(Tb)), having the result
Y(Tb1, t1) as well as the formation of correction factors
kYcal=Y(Tb1,t1)/Y(Tbcal,t0) which are valid for the board
temperature (Tbcal) measured during the calibration.
For the brightness calibration for an illuminating module of the
LED illuminating device a measurement of the brightness (Y) and the
board temperature (T.sub.b) for LED color immediately after turning
on, having the result Y(Tbcal, t0), a conversion to the brightness
in the static state at an assumed board temperature (Tb1) for each
LED color according to Y(T.sub.b1)=Y(Tbcal,t0)*kYcal is carried out
and the brightnesses (Y) of the LED colors in the LED illuminating
device converted to the assumed board temperature (Tb1) are stored.
For color calibration of the LED illuminating device, a measurement
of the spectrum is effected and brightness (Y) derived out of it as
well as chromaticity coordinates (x, y) for each LED color of the
LED illuminating device, a conversion of the brightness of the
spotlight to a board temperature (Tb1) by the characteristic line
(Y=f(Tb)) and scaling spectra to (Y=Y(T.sub.b1)), storing the
calibration data (x, y) and (Y(T.sub.b1)) for each LED color in the
LED illuminating device, a calculation of the optimum luminous flux
portions of the LED colors from the measured spectra for N color
temperature interpolation points using the program-controlled
processing unit, storing the luminous flux portions of the LED
colors for N color temperature interpolation points in the memory
of the LED illuminating device and/or storing the luminous flux
portions of the LED colors in table form dependent on the target
chromaticity coordinates (x, y).
Finally, a color control of the LED illuminating device under using
the stored calibration data for N color temperature interpolation
points and/or as chromaticity coordinates table for the luminous
flux portions of the LED colors, the temperature characteristic
lines for each color and the brightness (Y) and the chromaticity
coordinates (x, y) for each LED color can be effected by
determining the PWM control signals for the LED colors (PWM.sub.A)
for the desired chromaticity coordinates (x, y) and the desired
brightness (Y), measuring the board temperature (Tb), determining
the temperature-dependent PWM correction factors for each LED color
from the approximation characteristic lines (fPWM=1/Y) stored in
the memory, detecting the total power of the LED illuminating
device or the electrical current fed to the single LEDs of the LED
illuminating device and controlling the LEDs of the LED
illuminating device with the PWM correction factors at a total
power of the LED illuminating device or a electrical current fed to
the LEDs of the LED illuminating device smaller than the specified
maximum value (Pmax, Imax) or determining a cut-off factor
(kCutoff) for limiting the current or power for all LED colors from
kCutoff=Pmax/Pneu or kCutoff=Imax/Ineu and controlling the LEDs of
the LED illuminating device with new PWM factors according to
PWM.sub.T=PWMA*fPWM*kCutoff.
The precedingly described calculation steps for the determination
of the temperature-dependent spectra and the following new
calculations of the mixing ratios can be effected both "online"
within the spotlight and in advance in the context of the
calibration.
An apparatus for the temperature-dependent adjustment of the color
properties or the photometric properties of an LED illuminating
device having variously colored LED color groups, the luminous flux
portions of which determine the color of light, color temperature
and/or the chromaticity coordinates of the light mixture emitted
from the LED illuminating device is characterized by an input
device for adjusting the color of light, color temperature and/or
the chromaticity coordinates of the light mixture to be emitted
from the LED illuminating device and for specifying
application-specific target parameters and their admissible
deviations from an ideal value, a temperature measuring device
arranged within the housing of the LED illumination device and/or
in the area of at least one LED of the variously colored LED color
groups and emitting a temperature signal corresponding to the
measured temperature, a control device for controlling the LEDs of
the variously colored LED color groups with pulse-width modulated
current pulses, a memory having stored calibration data for each
LED color group for at least one value determining the emission
spectrum depending on the temperature and a microprocessor
connected to the control device and to the memory for determining
pulse-width modulated control signals corresponding to the luminous
flux portions for each LED color group for controlling the LEDs of
the LED color groups depending on the temperature signal provided
by the temperature measuring device.
The input device for adjusting the color of light, color
temperature and/or the chromaticity coordinates of the light
mixture to be emitted from the LED illuminating device and for
pre-setting application-specific target parameters and their
admissible deviations from an ideal value consists preferably of a
mixing device or DMX console.
The control device for controlling the LED color groups with
pulse-width modulated current impulses has a program-controlled
input connected to the microprocessor, a light mixing input
connected to the input device and a sensor and/or calibration input
connected to a sensor and/or a calibration handheld unit and is
connected to a feeding voltage source.
BRIEF DESCRIPTION OF THE DRAWINGS
The methods according to the invention and their respective
advantages are subsequently further explained by means of exemplary
embodiments. In the Figures:
FIG. 1 shows a schematic depiction of an LED illuminating device
designed as LED spotlight or LED panel of different size.
FIG. 2 shows a perspective depiction of an illuminating module
having a module carrier and a light source connected to the socket
of a module heat sink.
FIG. 3 shows a block diagram of a module electronics having
similarly constructed driver circuits;
FIG. 4 shows emission spectra of five variously colored LEDs of an
LED illuminating device.
FIG. 5 shows a graphic depiction of the temperature dependency of
LEDs of different color and material composition.
FIG. 6 shows a graphic depiction of the temperature dependency of
the peak wavelength of the LED color groups amber and red.
FIG. 7 shows a graphic depiction of the temperature dependency of
the half-width for the LED color groups amber and red.
FIG. 8 shows a graphic depiction of the temperature dependency of
the spectra for tungsten.
FIG. 9 shows a graphic depiction of the temperature dependency of
the spectra for daylight.
FIG. 10 shows a graphic depiction of the relative luminance for
tungsten and daylight dependent on the temperature.
FIG. 11 shows a graphic depiction of the color temperature shift
for tungsten and daylight dependent on the temperature.
FIG. 12 shows a schematic block diagram of a program-controlled
processing unit for determining the luminous flux portions or
pulse-width modulated signals of color groups of variously colored
LEDs.
FIG. 13 shows a schematic block diagram of the algorithm for the
color correction by a spectral approximation via the Gaussian
distribution without light sensor.
FIG. 14 shows a graphic depiction of the relative luminance over
the wavelength for the approximation of the emission spectra by the
Gaussian distribution for the color groups amber and blue.
FIG. 15 shows a schematic block diagram of the algorithm for the
color correction by spectral approximation via the Gaussian
distribution with a light sensor.
FIG. 16 shows a schematic block diagram of the algorithm for the
color correction by a spectral approximation via the Gaussian
distribution with light sensor and brightness compensation.
FIG. 17 shows a schematic block diagram of the algorithm for the
color correction by calculating temperature-dependent, optimized
mixing ratios for the color temperature settings.
FIG. 18 shows a schematic block diagram of the algorithm for
determining temperature-dependent dimming factors from stored
characteristic lines of the temperature-dependent mixing ratios of
the color temperature settings.
FIG. 19 shows a schematic block diagram of the algorithm for the
color correction by determining temperature-dependent dimming
factors from stored characteristic lines under consideration of
constant luminous flux portions without brightness sensor.
FIG. 20 shows a schematic block diagram of the algorithm for the
color correction by determining temperature-dependent dimming
factors from stored characteristic lines under consideration of
constant luminous flux portions with brightness sensor.
FIG. 21 shows a characteristic line for the relative brightness of
an LED color or LED color group dependent on the board temperature
T.sub.b for a color control by temperature characteristic
lines.
FIG. 22 shows a characteristic line for the relative brightness of
an LED color or LED color group dependent on the board temperature
T.sub.b for a color control by temperature characteristic
lines.
FIG. 23 shows a characteristic line for the relative brightness of
an LED color or LED color group dependent on the board temperature
T.sub.b for a color control by temperature characteristic
lines.
FIG. 24 shows an equivalent circuit diagram of the thermal
resistance between LED board and junction of the LED chips.
FIG. 25 shows a flow chart.
FIG. 26 shows a flow chart.
FIG. 27 shows a flow chart.
FIG. 28 shows a flow chart.
FIG. 29 shows a flow chart.
FIG. 30 shows a spectra for the clarification of the differences
between cold and warm spectra for the setting 3200 K.
FIG. 31 shows a spectra for the clarification of the differences
between cold and warm spectra for the setting 5600 K.
FIG. 32 shows the color temperature (CCT) deviation cold-warm
dependent on the color temperature.
FIG. 33 shows the chromaticity coordinates deviation dx, dy
(cold-warm) dependent on the target chromaticity coordinate x for
target chromaticity coordinates x, y along the Planckian locus in
the color temperature range between 2200 K and 24000 K.
FIG. 34 shows the optimum luminous flux portions warm and cold as
function of the color temperature CCT.
FIG. 35 shows a graphic of the measured color temperature of a
five-channel LED module dependent on the NTC temperature for the
setting CCT=3200 K with implemented correction of the spectral
shift.
FIG. 36 shows a graphic of the measured color temperature of an LED
module dependent on the NTC temperature for the setting CCT=5600 K
with implemented correction of the spectral shift;
FIG. 37 shows a flow-chart for determining the temperature
characteristic lines dependent on the dimming factor (PWM) and the
forward voltage.
FIG. 38 shows brightness-temperature characteristic lines for
yellow and red LEDs as well as a linear interpolation and
extrapolation for the yellow LED for +/-3 nm wavelength
deviation.
DETAILED DESCRIPTION
FIG. 1 shows a section through the schematic construction of an LED
illuminating device designed as LED spotlight 1 having
cylinder-shaped housing 10, in which an LED light source 3 is
arranged which is composed of a ceramic board, variously colored
LEDs arranged on the ceramic board in chip-on-board technology and
a pottant applied over the LEDs. The LED light source 3 is applied
directly onto a cooling body 11 made of well heat conducting
material like copper or aluminum by means of a heat conducting
adhesive, the heat sink 11 dissipating the heat emitted from the
LEDs of the LED light source 3. A fan 12 arranged on the backside
of the LED spotlight 1 provides for an additional cooling of the
LEDs.
The light mixing is effected by a cone-shaped or alternatively
cylinder-shaped light mixing rod 13 at the end of which a diffusion
disc 14 designed as POC foil is arranged. The LED spotlight 1 can
be adjusted continuously between a spot and flood position by a
Fresnel lens 15 which can be adjusted in the longitudinal direction
of the LED spotlight 1.
FIG. 2 shows a perspective depiction of an illuminating module
which consists of a quadrangular module carrier 2 designed as
conductor board on which a module electronics 5 is arranged and
which has a recess 21 through which a socket 110 of a module heat
sink 11 is plugged, the socket 110 projecting over the surface of
the module carrier 2, the module carrier 2 being connected to the
lower side of a connection plug board 16 via which the module
electronics is connected to a power controlling unit. A light
source 3 is arranged on the socket 110 of the module cooling body
16, the light source 3 having several LEDs 4 arranged on a
cubic-shaped metal core board, the LEDs 4 emitting light of
different wavelength and therewith color, the light source 3 also
having a temperature sensor 6 and conductor paths for connecting
the LEDs 4 and the temperature sensor 6 to the edges of the metal
core board, from where they are connected to the module electronics
via a direct wire or a bond connection.
The LEDs 4 are composed of several LEDs emitting light of different
wavelength, i.e. different color. By a close arrangement of the
LEDs 22 on the metal core board a light mixture of the different
colors is already generated, the light mixture being adjustable by
the choice of the LEDs and being able to be optimized by
additionally procedures like optical light focusing and light
mixing and to be kept constantly by further control and regulation
procedures independently on, e.g., the temperature to be able to
adjust a desired color temperature, brightness and the like.
FIG. 3 shows a functional diagram of the module electronics 5 for
controlling six LED groups having two LEDs 401, 402; 403, 404; 411,
412; 421, 422; 431, 432; 441, 442 in each case connected in series
and emitting light of the same wavelength for the regulation of the
light mixture to be emitted from the LEDs by a brightness control
of the single LED groups by a pulse-width modulated control voltage
and controlling a temperature-stabilized current source for feeding
the LED groups.
The module electronics 5 contains a microcontroller 50 which
provides six pulse-width modulated control voltages PWM.sub.1 to
PWM.sub.6 to six constant current sources 51 to 56 being
constructed identically. The microcontroller 50 is connected to an
external controller via a serial interface SER A and SER B and has
inputs AIN1 and AIN2 which are connected to a temperature sensor 6
and a brightness or color sensor 7 of the illuminating module via
amplifiers 60, 70.
The identically constructed current sources 51 to 56 are very well
temperature-stabilized and contain a temperature-stabilized
constant current source 57 which is connected to an output PWM1 to
PWM6 in each case of the outputs PWM1 to PWM6 providing the
pulse-width modulated control voltages of the microcontroller 50
and is connected to a feeding voltage U.sub.LED1 to U.sub.LED6 via
a resistor 59. The temperature-stabilized constant current source
57 is on the output side connected to the anode of the LEDs
connected in series of an LED group which emit light of the same
wavelength in each case and to the control connector of an
electronic switch 58 which on the one hand is connected to the
cathode of the LEDs connected series and on the other hand to the
ground potential GND.
The temperature-stabilized constant current source 57 is
characterized by a fast and neat switching at a switching frequency
of 20 to 40 kHz. To keep the power losses of the illuminating
module as small as possible, the LED chips being differently in the
production technology are fed with up to six different feeding
voltages U.sub.LED1 U.sub.LED6.
By arranging the temperature-stabilized current sources 51 to 56 on
the module carrier of the illuminating module the modularity of the
system is ameliorated and the voltage supply is simplified. By a
reduction of the different feeding voltages U.sub.LED1 to
U.sub.LED6 by an application of only two different voltages for a
group-wise grouped together voltage supply of the current sources
51 to 56 for, e.g., the red and yellow LEDs on the one hand and the
blue, green and white LEDs on the other hand, the illuminating
module needs only five interfaces, i.e. a connection of the
illuminating module via five conductors, namely two supply voltages
V.sub.LED1 and V.sub.LED2, ground potential GND and the serial
interfaces SER A and SER B with an external controller for the
higher ranking control and regulation of a plurality of likewise
constructed illuminating modules.
To clarify the different methods according to the invention for the
adjustment of the color properties or photometric properties of an
LED illuminating device and of the problem underlying the
invention, subsequently the different parameters which determine
the color emission of LEDs are explained in summary by means of
FIGS. 4 to 11.
FIG. 4 shows the spectra of variously colored LEDs in an LED
illuminating device as depiction of the relative luminance over the
wavelength of the light emitted by an LED illuminating device.
Since LEDs do not emit light monochromatically with a sharp
spectral line but in a spectrum having a certain bandwidth which
spectrum can be approximately assumed as Gaussian bell-shaped
curve, the emission spectra of LEDs can be simulated as a Gaussian
distribution. FIG. 4 shows in continuous line the emission spectrum
of a white LED, in short dashed line the emission spectrum of a
blue LED, in long dashed line the emission spectrum of a yellow or
amber colored LED, in dotted line the spectrum of a red LED and in
a dotted and dashed line the emission spectrum of a green LED.
It can be learnt from this spectral depiction that the shape of the
spectrum of the LED emitting white light differs strongly from the
spectra of the LEDs emitting colored light. This results from the
technology of generating white light in which as basis for the
light generation a blue chip is used, the spectrum of which is the
reason for the first small peak of the spectrum of the white LED.
The phosphor covering of the blue LED chip converts the blue light
partially into yellow light from which the second, higher peak in
the yellow area of the spectrum results. In mixed form, the
portions result in white light. By the thickness of the phosphor
covering, the color temperature of the white light can be varied so
that in this manner both warm white and daylight white LEDs can be
produced.
FIG. 5 shows the temperature dependency of LEDs in a depiction of
relative luminance over the junction temperature T in .degree. C.
at different material combinations. The temperature dependency of
the LEDs is making up big problem when using LEDs as illuminant.
With increasing junction temperature T the properties and
characteristics of LEDs vary significantly. Thus, the luminance
strongly decreases with increasing temperature T and a shift of the
spectra to higher wavelengths, i.e. towards red light, occurs.
These temperature dependencies are differently strong pronounced
dependent on the used materials, resulting in the fact that also
the colorimetric properties of a light composition mixed from LEDs
additively emitting white light and colored light vary.
Subsequently the luminances, peak wavelengths and half-widths of
single LED color groups being composed of several LEDs emitting
light of the same color shall be regarded dependent on a
temperature present at an LED of the respective color group by
means of FIGS. 6 to 11 and an analysis of the spectra and the
luminances as well as the color temperature and the chromaticity
coordinates of the light mixtures for tungsten and daylight, also
dependent on the present temperatures, shall be carried out.
As can been seen from the depiction according to FIG. 5 the
variously colored LEDs have a differently strong temperature
dependency. Those LEDs which emit in the long-wave range of the
visible spectrum decrease in the luminance with increasing
temperature T in .degree. C. significantly stronger than those LEDs
which emit in the short-wave range of the visible spectrum. Thus,
the LED colors amber and red show a luminance decrease of 128% or
116% at 20.degree. C. to 65% or 75% of the initial value at
60.degree. C. The color groups blue and green are significantly
less temperature-dependent with respect to their luminance. Since
the white LEDs are based on the technology of blue LEDs, also a
significantly smaller temperature dependency of the luminance
decrease of white LED results.
Like in case of the luminance, the temperature dependency also
differs for the peak wavelength for different LED types.
FIG. 6 exemplarily shows the temperature dependency of the peak
wavelength .lamda..sub.P for the LED groups amber and red and
clarifies a shift of the peak wavelength .lamda..sub.P with
increasing ambient or junction temperature T in .degree. C. of the
LEDs. Also with respect to the peak wavelength .lamda..sub.P the
LEDs in the higher-wave visible range like amber and red are
stronger temperature-dependent than LEDs of the LED groups blue and
green which are much less temperature-dependent.
Also the half-width w.sub.50 of the emitted spectra is linearly
dependent on the temperature T in .degree. C. as are the luminance
and the peak wavelength .lamda..sub.P of the single LED color
groups. In contrast to those two latter-mentioned parameters, the
differences between the various LED color groups are here not so
serious. FIG. 7 exemplarily depicts the devolutions of the
half-width w.sub.50 of the LED colors amber and red over the
temperature T in .degree. C. In contrast to the luminance and peak
wavelength .lamda..sub.P, the half-width w.sub.50 is for the LEDs
of the groups blue and green comparably temperature-dependent like
for the groups amber and red.
For an explanation of the temperature dependency of the spectra for
the light mixtures "tungsten" and "daylight", FIG. 8 depicts the
relative luminance over the wavelength in nm for the light mixture
"tungsten" and FIG. 9 depicts it for the light mixture "daylight"
at different junction temperatures.
A significant decrease of the luminance with the temperature can be
seen for both light mixtures, wherein the spectrum of the light
mixture shifts towards longer wavelengths due to the shift of the
peak wavelength of the single LED color groups. The strong
luminance decrease of the LED color groups amber and red is
particularly obvious in FIGS. 8 and 9.
FIG. 10 shows the relative luminance in percent over the
temperature T in .degree. C. of the light mixtures "tungsten" and
"daylight" relating to an ambient temperature of 20.degree. C. and
clarifies that the temperature influence onto the single LED color
groups causes a decrease of the luminance in the light mixture
which is non-negligible. Thereby, the light mixture "tungsten"
shows a bigger relative luminance decrease than the light mixture
"daylight".
FIG. 11 shows the color temperature shift dCCT in K for "tungsten"
and "daylight" dependent on the ambient temperature T and clarifies
that the significantly stronger temperature sensitivity of the LEDs
in the ranges red and amber with respect to the luminance leads to
a blue shift of the color of light with increasing temperature.
To correct for the precedingly described temperature-dependent
modifications of the chromaticity coordinates, different methods
can be applied according to the invention. Firstly, the spotlight
has to be calibrated by determining a basic mixture for the
settings "tungsten" with 3200 K and "daylight" with 5600 K. To
adjust the correct color of light at the spotlight, the portions,
i.e. the pulse-width of the pulse-width modulation (PWM) have to be
determined for the control of the LED color groups. These portions
are calculated with the aid of a program-controlled processing unit
schematically depicted in FIG. 12.
To be able to adjust the correct color of light at the spotlight,
the portions (pulse widths .tau.) of a pulse-width modulation (PWM)
have to be determined for all LED color groups. This is calculated
with the aid of the program-controlled processing unit, the
principle construction of which is depicted in FIG. 13.
Description Block Diagram LED Mix
Different spectra of LED colors can be read into the
program-controlled processing unit provided within the solution of
the preceding problem, e.g. the LED colors red, blue, yellow, white
and amber indicated in FIG. 12. The user can adjust the following
optimization parameters as set values on the input side: the target
color temperature of the LED mixture (e.g. 3200 K, 5600 K) the film
material or the camera sensor with which no color deviation shall
be produced as compared to the reference illuminant (good
mixed-light capability), (e.g. Kodak 5246D, Kodak 5274T) the
reference illuminant for the camera (e.g. incandescent lamps 3200
K, daylight 5600 K, HMI etc.) for which a good mixed-light
capability shall be achieved.
The program-controlled processing unit optimizes the mixture
portions of the imported color spectra of the LED colors onto the
following parameters via genetic algorithms: color temperature
minimum distance from the Planckian locus (i.e. as possible, no
color deviation in the direction green or magenta is visible for
the eye) color rendering index (as close to 100 as possible)
mixed-light capability with film or digital camera. The color
distance between the determined mixture and the reference
illuminant has to be minimal for the recording medium film or
camera.
Besides the set values, the user can enter admissible deviations or
tolerances .DELTA. CCT (K), .DELTA. C_Planck (color distance to the
Planckian locus), .DELTA. CRI, .DELTA.C_film (color distance
mixed-light capability) for the precedingly indicated target values
CCT (K), film material/type of sensor and reference illuminant for
mixed-light capability.
The portions of the LED spectra of the LED colors for adjusting an
optimum mixture having being entered into the program are then the
result of the optimization by the program-controlled processing
unit. The output of the LED mixture, i.e. the dimming factors and
the luminous flux portions for each of the LED colors as well as
the colorimetric values achieved with this mixture for the
chromaticity coordinate, the color temperature, the color distance
to the Planckian locus, the color rendering index as well as the
mixed-light capability with a film camera or a digital camera are
also calculated and output.
For tracking the spectra of the single LED colors or LED color
groups of a light mixture dependent on the housing-internal ambient
temperature, the board or the junction temperature of the LED
chips, different methods can be applied according to the invention
which are subsequently explained by means of FIGS. 13 to 20.
FIG. 13 shows a first variant in which the control of the LEDs of
the single LED colors is effected online with a pulse-width
modulation (PWM), i.e. by immediate input of the
temperature-dependently determined dimming factors for the single
LED colors at the control electronics of the LEDs or in which the
luminous flux portions being necessary for the light mixture for
each of the LED colors are output. In this first method no light
sensor is used for the luminance measurement.
The calibration data, i.e. the characteristic lines for the peak
wavelength peak=f(T), the half-width w.sub.50=f(T) and the
luminance Y.sub.0=f(T) as function of the temperature are stored in
the microprocessor of the program-controlled processing unit as
function or table in the memory of the microprocessor for each LED
color. After the start of the program, the following is effected:
1. Measuring the temperature at an LED or an LED color group, 2.
Determining the temperature-dependent parameters for the peak
wavelength peak=f(T), the half-width w.sub.50=f(T) and the
luminance Y.sub.0=f(T) from the stored characteristic lines,
calculation of the new spectra via the Gaussian distribution
according to the Gaussian bell-shaped curve
.function..lamda.e.lamda..lamda. ##EQU00003## or for an even more
precise approximation of the spectrum via the formula
.function..lamda..times..pi.e.times..lamda..lamda. ##EQU00004##
being based on the Gaussian distribution, with .lamda..sub.p the
peak wavelength of the LED emission spectrum, w.sub.50 the
half-width of the LED emission spectrum and f.sub.L a
temperature-dependent conversion factor 3. Importing the spectra
into the program-controlled processing unit and calculating the new
dimming factors adapted to the temperature being modified with
respect to the initial temperature for the new light mixture from
the spectral approximation via the Gaussian distribution, 4.
Setting dimming factors corresponding to the new light mixtures at
the LEDs of the single LED color groups of the spotlight via the
control electronics for controlling the LEDs of each LED color
group.
The program loop is being closed after controlling the LEDs by a
new temperature measurement.
FIG. 14 shows a graphic depiction of the relative luminance over
the wavelength during the approximation of the emission spectra by
the Gaussian distribution for the color groups amber and blue and
shows a very good approximation to the measured values in each
case.
In case of an additional use of a light sensor for the luminance
measurement, the program depicted as flow-chart in FIG. 15 is used
in which the program step 5. Luminance measurement with light
sensor and dimming the spotlight onto the set value. is added to
the precedingly described program steps 1 to 4.
The calibration data, i.e. the characteristic lines for the peak
wavelength peak=f(T), the half-width w.sub.50=f(T) and the
luminance Y.sub.0=f(T), are stored as function of the temperature
in the memory of the microprocessor for each LED color as function
or table also in case of the program depicted as flow-chart in FIG.
15. After the start of the program, a measurement of the
brightnesses or luminance Y.sub.0=f(T) is effected for each LED
color group of the single LED colors of the spotlight. In the next
program step, a temperature measurement of the housing-internal
ambient temperature of the LEDs follows, i.e. of the board or
junction temperature of the LEDs of the spotlight. From these
measurement values the temperature-dependent factors Y.sub.0=f(Tu)
are determined from the memory connected to the microprocessor and
subsequently the correction factors are calculated by the quotient
fK=Y.sub.0(T.sub.u)/Y.sub.t(T.sub.u) with the initial brightness
Y.sub.0 and the brightness Y.sub.t at the temperature T, which
correction factors represent the relative luminance decrease over
the whole temperature range and indicate a temperature-dependent
conversion factor of the luminance of the spectrum relatively to
the luminance of the initial spectrum. This is followed by an anew
temperature measurement as next program step, and the
temperature-dependent factors for the peak wavelength peak=f(T),
the half-width w.sub.50=f(T) and luminance Y.sub.0=f(T) are
determined from the stored characteristic lines. Analogously to the
flow chart depicted in FIG. 13 subsequently a spectral
approximation is effected by the Gaussian distribution.
In the subsequent program step, the spectra for each color group
being approximated by the Gaussian distribution are multiplied by
the color-dependent correction factors fk determined according to
the preceding formula. Subsequently, the dimming factors for the
pulse-width modulation of the single LEDs of the LED color groups
of the spotlight are determined for the light mixture at the
measured temperature with the aid of the program-controlled
processing unit depicted in FIG. 12 and the single LEDs of each LED
color group of the spotlight are controlled by the control
electronics with the calculated dimming factors. Also in case of
this program procedure, the program loop is closed by a following
anew temperature measurement.
The illuminating device can be adjusted to the new calculated light
mixture with the aid of this program procedure and the color
correction is effected as a result of the modified housing-internal
ambient temperature, board or junction temperature. To correct
possible deviations in the luminance which can occur after the
correction, a luminance measurement is effected with a light or a
V(.lamda.) sensor with the aid of which the difference between the
actual value and the set value of the luminance is determined and
the illuminating device is adapted by evenly dimming all color
groups to the set value.
The advantage of the control program depicted in FIG. 15 is that a
compensation of aging effects is possible since a temporal
brightness decrease is detectable by the light sensor provided
within this control program. If an RGB sensor or color sensor or a
spectrometer is used as sensor element instead of a light sensor or
a V(.lamda.) sensor, also color modifications of the single LED
colors of the spotlight can be detected additionally to the
brightness modifications.
A further variation exists in additionally detecting modifications
of the peak wavelength peak=f(T) and the half-width w.sub.50=f(T)
in case of arranging an RGB sensor or color sensor or a
spectrometer.
The flow-chart depicted in FIG. 16 serves for explaining a control
program for controlling the LEDs of different LED color groups of a
spotlight with a brightness correction of the temperature-dependent
light mixture using a light sensor.
Also in case of this control program, the storage of calibration
data in the microprocessor for each LED color as a function or
table for the temperature-dependent parameters peak wavelength
peak=f(T), half-width w.sub.50=f(T) and luminance Y.sub.0=f(T) is
necessary. After the program start, the actual brightnesses Yt is
measured for each LED color group. This is followed by a
measurement of the housing-internal ambient temperature or the
board or junction temperature Tu. Subsequently, the
temperature-dependent factors Y.sub.0=f(Tu) are determined from the
memory connected to the microprocessor and the correction factors
fk are calculated out of it according to the quotient
fk=Y.sub.0(T.sub.u)Y.sub.t(T.sub.u) with the initial brightness
Y.sub.0 and the brightness Y.sub.t at the temperature T.
After the calculation of the correction factors fk, an anew
temperature measurement is effected which forms the basis for the
determination of the temperature depending factors for the peak
wavelength peak=f(T), the half-width w.sub.50=f(T) and luminance
Y.sub.0=f(T) from the stored characteristic lines. Like in case of
the precedingly described control programs, subsequently a spectral
approximation is effected by the Gaussian distribution. This is
followed by a multiplication of the spectra with the
color-dependent correction factors fk for which the new light
mixture i.e. new set values for the dimming factors and luminous
flux portions for the LEDs of the LED color groups of the spotlight
are calculated in the subsequent program step with the aid of the
program-control processing unit depicted in FIG. 12. The LEDs of
the LED spotlight are controlled by the new dimming factors for the
new light mixture in an online operation.
After controlling the LEDs with the new dimming factors, an anew
brightness measurement is effected for detecting the actual value
Y.sub.Ist individually for each LED color group with the aid of the
light sensor or V(.lamda.) sensor. A correction factor
f=Y.sub.Ist/Y.sub.Soll is calculated from the measurement of the
actual value Y.sub.Ist of the brightness measurement and the
specified set value for the brightness Y.sub.Soll, and subsequently
the LEDs are controlled with new dimming factors which result from
the quotient of the calculated dimming factors and a correction
factor f=Y.sub.ist/Y.sub.soll according to the relation PWM
factors(new)=PWM factors(calculated)/f.
Also in case of this control program, the program loop is closed
with an anew temperature measurement. Additionally, a compensation
of aging effects can be provided by detecting a temporal brightness
decrease by a light sensor or a V(.lamda.) sensor. When using an
RGB sensor or color sensor or spectrometer as sensor element,
additionally color modifications of the single LED colors of the
spotlight can be detected besides brightness modifications, and
additionally modifications of the peak wavelength peak=f(T) and the
half-width w.sub.50=f(T) can be detected.
FIG. 17 shows a flow-chart for the calibration of an LED spotlight
which results in a multi-dimensional table for the pre-calculation
of the mixing ratios of the light mixtures of several LED colors at
different temperatures, wherein this calculation is effected in
advance outside the spotlight.
After the start of the calibration program, one has to decide if an
approximation via a Gaussian distribution is desired. If the
approximation is to be effected via the Gaussian distribution, the
temperature-dependent parameters of the peak wavelength peak=f(T),
the half-width w.sub.50=f(T) and the brightness or luminance
Y.sub.0=f(T) for each LED color is determined or measured. Out of
it, a spectral approximation by the Gaussian distribution is
effected over the whole temperature range of the spotlight
application.
Alternatively, a measurement of the temperature-dependent spectra
of the LED colors is performed instead of an approximation by the
Gaussian distribution.
The temperature-dependently optimized light mixtures of the single
used LED colors are calculated from the results of both
alternatives with the aid of the program-controlled processing unit
depicted in FIG. 12, i.e., the dimming factors for the single LEDs
of the LED color groups for N0 color temperatures, e.g. for
daylight, tungsten and optionally for additional color temperature
interpolation points. This calculation in followed by storing the
temperature-dependent mixtures ratios, i.e. the dimming factors for
the single LEDs of the LED color groups of the spotlight for the N0
color temperature settings. These N0 color temperature settings can
then form the basis for a control program for the regulation of the
color temperature of the spotlight according to the flow-chart
depicted in FIG. 18.
FIG. 18 requires the determination and storage of calibration data
in the microprocessor of the control electronics for the LEDs of
the single LED color groups of the spotlight for N0 color
temperature interpolation points in form of a function or in form
of a function or table stored in the memory of the microprocessor,
from which the mixing ratio results, i.e. the dimming factors as
function of the ambient temperature Tu and the color temperature
CCT.
After the start of the control program, a measurement of the
housing-internal ambient temperature or the board or junction
temperature of the LEDs, the LED color groups or single LEDs of
each color group is effected. The temperature-dependent dimming
factors are determined from the actual value of the temperature
measurement from the characteristic lines stored in the memory of
the control electronics, and the LEDs of the single LED color
groups are controlled with the temperature-dependent new dimming
factors. Also in case of this control program, the program loop is
closed with an anew temperature measurement.
FIGS. 19 and 20 depict flow-charts for two further control methods
for the determination of dimming factors for the
temperature-dependent light mixtures of the LED color groups of an
illuminating device without and with the application of a luminance
measurement with a light sensor or a V(.lamda.) sensor.
FIG. 19 shows the procedure of a control program which is based on
the adjustment of constant luminous flux portions of the single LED
color groups of the illumination device without effecting a
luminance measurement with a light sensor or a V(.lamda.) sensor.
Calibration data are stored in the memory of the control
electronics as function or table, namely the characteristic line
for the brightness Y=f(Tu) for each LED color of the LED color
groups of the illuminating device and the interpolation points for
the respective mixing ratio in form of dimming factors as function
of the color temperature CCT.
After the start of the program, a temperature measurement is
effected which forms the basis for determining the
temperature-dependent factors Y=f(Tu) for the single LED color
groups from the stored characteristic lines. The respective dimming
factors are calculated by an according normalization from the
determined temperature-dependent factors Y according to the
equation PWM(T.sub.u)=PWM(T.sub.0)/Y(T.sub.u) with T.sub.0 being
the initial or basis temperature and T.sub.u being the actual
measured temperature. The single LEDs of each LED color group of
the spotlight are controlled by the dimming factors PWM(T.sub.u)
calculated in this way dependently on the actual temperature, and
the program loop is closed by an anew temperature measurement.
The determination of temperature-dependent light mixtures of the
single LEDs of the LED color groups of the spotlight taking
constant luminous flux portions as a basis can additionally be
linked with a luminance measurement by a light sensor or a
V(.lamda.) sensor.
FIG. 20 shows a flow-chart of a control program for determining the
dimming factors for the single LEDs of several LED color groups of
a spotlight with a temperature measurement and additionally a
luminance measurement by a light sensor or a V(.lamda.) sensor.
Also in case of this embodiment, the calibration data of the
brightness Y and the interpolation points for the mixing ratio
stored as function or table in the memory of the microprocessor of
a control electronics are imported in the form of dimming factors
as function of the ambient temperature Tu and of the color
temperature CCT of the LEDs of the single LED color groups of the
illuminating device. After the start of the program, a measurement
of the housing-internal ambient temperature or the board or
junction temperature T.sub.u of the LEDs, the LED color groups or
single LEDs of each LED color group is effected. The
temperature-dependent factors Y=f(Tu) are determined from the
actual values of the temperature measurement from the stored
characteristic lines and the LEDs of the single LED color groups
are controlled by the calculated temperature-dependent new dimming
factors PWM(T.sub.u)=PWM(T.sub.0)/Y(T.sub.u)
In contrast to the control method precedingly described by means of
the flow-chart depicted in FIG. 19, no anew temperature measurement
is effected after controlling the LEDs of each LED color group with
the new dimming factors, but firstly a luminance measurement is
effected with the aid of the light sensor or the V(.lamda.) sensor,
which measurement is followed by a calculation of the correction
factors f=Y.sub.Ist/Y.sub.Soll. Taking these correction factors f
as basis, the control of the LEDs of each LED color group of the
spotlight is effected with new dimming factors according to the
equation PWM factors(new)=PWM factors(calculated)*f
In case of this control method, the control of the LEDs with new
dimming factors inserted after the calculation of the new dimming
factors taking the temperature-dependent factors Y=f(Tu) as a basis
from the stored characteristic lines can be omitted and instead the
luminance measurement with the light sensor or the V(.lamda.)
sensor can be performed after calculating the dimming factors
according to the equation PWM(Tu)=PWM(T.sub.0)/Y(Tu).
Additionally, further data can be stored in the memory like, e.g.,
calibration data, data for warm and cold, luminous efficacies for
the set and the like which will be described in the following in
more detail.
In FIGS. 21 to 23 and 25 to 29 flow-charts and characteristic lines
for the relative brightness of an LED color or an LED color group
depending on the board temperature T.sub.b are depicted for a
further method for the color stabilization of an LED illuminating
device in which method the color control is effected by temperature
characteristic lines.
In case of this method, it is assumed that the brightness of the
LEDs of the single LED colors depends on the junction temperature
of the LEDs or on the measured board temperature Tb which is
measured instead of the difficultly measureable junction
temperature on a board on which LEDs emitting light of different
wavelength or color are arranged to a light source emitting mixed
light being controlled by a module electronics which is arranged
together with a board on a module carrier and forms together with
the board an illuminating module which can be grouped together with
a plurality of further illumination modules to an LED panel.
A) the Brightness of LEDs as Function of the Board Temperature
Tb
The dependence of the brightness Y of the LEDs of the LED
illuminating device on the junction temperature or on the measured
board temperature Tb is approximated by an approximation function
which is designed according to the desired degree of accuracy as
linear function having the form Y(Tb)=a+b*Tb as second-degree
polynomial having the form Y(Tb)=a+b*Tb+c*Tb.sup.2 (formula 1) or
as third-degree polynomial having the form
Y(Tb)=a+b*Tb+c*Tb.sup.2+d*Tb.sup.3
The quality of approximation is already very good in case of a
quadratic approximation function with a second-degree polynomial as
is proven by the diagram depicted in FIG. 21 for the LED color
amber which has the strongest temperature dependency together with
the LED color red.
The measured characteristic lines of the relative brightness Y(Tb)
as function of the board temperature T.sub.b in .degree. C. show a
curve shape depending on the current or power. In all cases, the
curve shape is this steepest for higher LED powers. This effect can
be detected both in case of a direct-current and a pulse-width
modulated PWM control of the LEDs as can be seen from the diagram
depicted in FIG. 22 from which the relative brightness in percent
over the board temperature Tb in .degree. C. can be extracted at
different dimming factors and therewith different currents.
This effect can be traced back to the fact that the temperature
sensor detecting the board temperature in praxis is located near to
the LED chip on the LED board of the light source of an
illuminating module as close as possible at the light-emitting LED
chips. Despite this proximity of the temperature sensor to the
light-emitting LED chips, there is a thermal resistance between the
site of temperature measurement and the junction of the LED chips
so that the measured temperature value is always lower than the
junction temperature. Thereby, the temperature difference depends
for each LED chip on the thermal power to be dissipated from the
respective LED chip and therewith on the LED power taken up. Since
thus the brightness of the LEDs emitting light of different
wavelength depends on the junction temperature, but the
characteristic lines are only recorded dependently on the board
temperature, the measured characteristic lines of the brightness as
function of the board temperature show a current-dependent or
power-depended curve shape.
From this the problem arises that the characteristic lines of the
brightness Y as function of the board temperature Tb depend on the
current of or on the power taken up by the single LEDs or LED color
groups so that a brightness correction with the precedingly
indicated formula 1 in which the dependency of the brightness of
the LEDs on the board temperature is approximated by a quadratic
approximation function is afflicted with systematic errors for
differing LED currents or thermal powers and would not work
optimally. This effect would occur, e.g., during dimming, i.e.
during the pulse-width modulated control of the LED illuminating
device.
An amelioration of the method to perform the brightness correction
on the basis of temperature characteristic lines Y=f(T.sub.b) can
be achieved in that the preceding formula 1 is amended as follows:
Y(Tb)=a+b(Tb+.DELTA.T)+c(Tb+.DELTA.T).sup.2 (formula 2)
A temperature correction value .DELTA.T is inserted into the
quadratic approximation function Y=f(T.sub.b) which temperature
correction value considers the modifications of the temperature
difference between the temperature sensor and the junction of the
LED due to modified thermal powers. This form can especially have
advantages as compared to a second-degree polynomial (formula 1) if
also the electronics has an (unwanted) temperature-dependent
behavior and the LED current additionally depends on the
temperature.
The correction value .DELTA.T thereby depends on the thermal
resistance between the temperature sensor and the junction of the
LEDs as well as on the thermal power or electric power of the LEDs
to be momentarily dissipated. It can either be calculated from
these parameters, if known, or be determined from series of
measurements with different electric powers.
In case of a known thermal resistance between the board and the
junction of the LED, the current-dependent correction value
.DELTA.T can be calculated from the LED currents as follows:
Rw=.DELTA.T/Pw with Rw being the thermal resistance between the
board and the junction, Pw being the amount of heat to be
dissipated which approximately corresponds to the LED power and
.DELTA.T being the temperature difference between board and
junction. From this follows .DELTA.T=Rw*Pw with the thermal power
Pw which approximately corresponds to the LED power
U.sub.LED*I.sub.LED.
The temperature correction value .DELTA.T has to be individually
considered for each LED color like the parameters a, b and c. The
current-dependent thermal power of the LEDs is determined by the
microprocessor form the values U.sub.LED*I.sub.LED. Since in case
of LEDs a part of the total power is converted into light, the
thermal power of the LEDs is always smaller than the product U*I.
This can be considered by an additional factor fw
Pw=fw*U.sub.LED*I.sub.LED
The color-dependent correction value .DELTA.T can be calculated
accordingly as follows: .DELTA.T=Rw*fw*I.sub.LED*U.sub.LED
In this manner, the behavior of the brightness Y measured in each
case dependent on the board temperature T.sub.b can be
reconstructed very well as is shown by the diagram depicted in FIG.
23 for the example of a yellow LED.
B) the Current Dependency of the Characteristic Lines
The measured characteristic lines of the brightness Y(Tb) as
function of the board temperature Tb shows according to FIG. 22 a
current-dependent or power-dependent curve shape. In all cases, the
curve shape is the steepest for higher LED powers. This effect can
be observed both for a direct-current control and for a PWM control
of the LEDs and both for AlInGaP materials and to a lower extent
for InGaN materials.
This effect can be traced back to the fact that the temperature
sensor is located for practical reasons close to the LEDs on the
LED board, as close as possible at the light-emitting chips.
However, there is a thermal resistance between the temperature
measurement point and the junction of the chips. The measured
temperature value is therefore always smaller than the junction
temperature. The temperature difference thereby depends for each
chip on the thermal power to be dissipated from each chip and
therewith on the LED power taken up, as can be seen from the
equivalent circuit diagram of the thermal resistance between LED
board and junction of the chips according to FIG. 24.
Since the brightness of the LEDs depends on the junction
temperature, the characteristic lines, however, have only been
recorded dependently on the board temperature, the measured
characteristic lines of the brightness as function of board
temperature show a current-dependent or power-dependent curve
shape.
From the preceding conclusion that the characteristic lines of the
brightness as function of the board temperature depend on the
current or on the total power taken up, it results that a
brightness correction according to formula 2 for deviating LED
currents or thermal powers is afflicted with systematic errors and
would not work optimally. This effect would, e.g., occur in case of
dimming the LED spotlight.
An amelioration of the method of the brightness correction on the
basis of temperature characteristic lines Y=f (Tboard) can be
achieved by amending formula 2 as follows:
Y(Tb)=A+B*(Tb+.DELTA.T)+C*(Tb+.DELTA.T).sup.2+D*(Tb+.DELTA.T).sup.3
formula 3
A temperature correction value .DELTA.T is inserted into the
quadratic or cubic approximation function Y=f(Tb) which temperature
correction value considers the modifications of the temperature
difference between the temperature sensor and the junction on the
basis of modified thermal powers.
The correction value .DELTA.T thereby depends on the thermal
resistance between sensor and junction as well as on the thermal
power to be momentarily dissipated or electric power of the LED
module. It can be either calculated from these parameters, if
known, or determined by series of measurements with different
electric powers.
In case of a known thermal resistance (board-junction) of the LED,
the current-dependent correction value .DELTA.T can be calculated
from the LED currents as follows: Rw=.DELTA.T/Pw Rw: thermal
resistance between board and junction Pw: amount of heat to be
dissipated, approximately LED power .DELTA.T: temperature
difference between board and junction .DELTA.T=Rw*Pw Pw: thermal
power, approximately corresponding to LED power
U.sub.LED*I.sub.LED
The temperature correction value .DELTA.T has to be individually
considered for each LED color like the parameters A, B, C and
D.
The current-dependent thermal power of the LEDs is determined by
the microprocessor from the values U.sub.LED*I.sub.LED. Since a
part of the total power of LEDs is converted into light, the
thermal power of the LEDs is always smaller than the product U*I.
This can be considered by additional factor fw:
Pw=fw*U.sub.LED*I.sub.LED
The color-dependent correction value .DELTA.T can thus be
calculated as follows: .DELTA.T=Rw*fw*I.sub.LED*U.sub.LED formula
4
In this manner, the measured behavior can be reconstructed very
well as is shown in the graphic depicted in FIG. 23 for the example
of a yellow LED.
The brightness-temperature characteristic lines are normalized to a
"working temperature" Tn which, e.g., represents the typical
operation temperature in the warm state.
Y(Tb)=A+B*(Tb+.DELTA.T-Tn)+C*(Tb+.DELTA.T-Tn).sup.2+D*(Tb+.DELTA.T-Tn).su-
p.3 formula 5
If the curves are normalized such that Y(Tb) becomes "1" for the
working temperature Tn then the parameter A results always in "1".
Therewith, the storage of this parameter in the memory can be
omitted.
The polynomial parameters A to D are determined with usual methods
of mathematics by means of curves recorded for different dimming
degrees of brightness as function of the board temperature for the
virtual characteristic line extrapolated to PWM=0.
To practically determine the correction value .DELTA.T without
considering the forward voltage, the thermal resistance Rw as well
as the correction factor fw are necessary to determine the thermal
power according to formula 4. Often, these values are not known.
Since the thermal power of the LEDs is directly proportional to the
electric power of the LEDs and therewith directly proportional to
the dimming factor of the LEDs, formula 4 can be rewritten as
follows: .DELTA.T.about.PWM .DELTA.T=E*PWM formula 6 with PWM being
the dimming factor between (0 . . . 1) and the power parameter
E.
If the polynomial parameters A to D as well as the power parameter
E are known, the relative brightness of the LED colors can be
calculated during the operation of the spotlight by formulae 5 and
6 from the actual values of the board temperature Tb as well as
from the individual LED dimming factors PWM:
Y(Tb)=A+B*(Tb+.DELTA.T-Tn)+C*(Tb+.DELTA.T-Tn).sup.2+D*(Tb+.DELTA.T-Tn).su-
p.3 with .DELTA.T=E*PWM
For practically determining the correction value .DELTA.T under
considering the forward voltage, the typical forward voltage
tolerances of LEDs lead to the fact that different LEDs of the same
type and the same color are operated with different LED powers even
if they are controlled with the same current and the same PWM. The
consideration of the individual forward voltages consequently leads
to a further amelioration of the quality of the applied temperature
characteristic line. From formula 4 it follows:
.DELTA.T.about.PWM*U.sub.LED .DELTA.T=E.sub.1*PWM*U.sub.LED formula
7
The parameter E1 can be determined from the value E determined for
formula 6 by dividing E by the forward voltage U.sub.Fref of the
LED module used for its determination.
The relative brightness of the LED colors can then be calculated
during the operation of the spotlight with formulae 5 and 7 from
the actual values of the board temperature Tb as well as from the
individual LED dimming factors and forward voltages:
Y(Tb)=A+B*(Tb+.DELTA.T-Tn)+C*(Tb+.DELTA.T-Tn).sup.2+D*(Tb+.DELTA.T-Tn).su-
p.3 with .DELTA.T=E.sub.1*PWM*U.sub.LED
To keep the brightness of the individual LED colors during the
operation of the spotlight constant, the PWM control signals are
multiplied with the temperature correction factor kT=1N(Tb)
dependent on the board temperature, the PWM as well as optionally
the forward voltage: PWM=PWM*kT=PWM/Y(Tb) formula 8
Precedingly:
Y(Tb) denotes the relative brightness depending on the board
temperature
Tb denotes the board temperature in .degree. C.
Tn denotes the working temperature in .degree. C.
.DELTA.T denotes the power-dependent temperature correction value
in .degree. C.
A . . . D denote polynomial coefficients
E, E.sub.1 denote power parameters
PWM denotes a PWM control signal (0 . . . 1)
Rw denotes the thermal resistance in K/W
U.sub.LED denotes the forward voltage in V
I.sub.LED denotes the LED current in A
Pw denotes the thermal power in W
fw denotes a correction factor.
The procedure of the method for the color control of LEDs emitting
light of different wavelength or color by temperature
characteristic lines can be extracted from the flow-charts depicted
in the FIGS. 25 to 29.
The flow-chart depicted in FIG. 25 serves for the determination of
temperature characteristic lines of an LED module, wherein the
determination of temperature characteristic lines is performed
randomly. The determined characteristic lines are then transferred
onto all LED modules and stored in their memory. A conversion
(interpolation/extrapolation) of the characteristic line parameters
onto the individual dominant wavelengths can be considered before
the storage, said conversion being subsequently explained.
In a first step, the brightness Y is measured dependently on
different board temperatures T.sub.b for each LED color at a
specified current in the steady state, and the characteristic line
Y=f(T.sub.b) is determined. In a second step, the characteristic
lines are normalized onto an arbitrarily chosen temperature value
close to the later working point T.sub.b1, i.e. Y(T.sub.b1)=1 is
determined.
In a third step, the parameters a and b are determined according to
the choice of the approximation function for a linear approximation
function having the form Y(Tb)=a+b*Tb for a quadratic approximation
function, i.e. a second-degree polynomial having the form
Y(Tb)=a+b*Tb+c*Tb.sup.2 or for an approximation function with a
third-degree polynomial having the form
Y(Tb)=a+b*Tb+c*Tb.sup.2+d*Tb.sup.3
The parameters a and b or a, b, c or a, b, c, d are stored in the
LED modules, in a central control device of the LED illuminating
device or in an external controller.
The flow-chart depicted in FIG. 26 shows the random determination
of calibrating correction methods for the LED modules which methods
are needed during the operation of the LED illuminating device for
a fast individual brightness calibration of the LED modules. The
calibrating correction factors describe the factor of the
brightness in the steady state with respect to the brightness
measuring value shortly after switching-on the LED illuminating
device and are determined randomly for each LED color.
In a first step for determining the calibrating correction factors
for each LED module, the brightness Y is measured dependently on
the board temperature T.sub.bcal for each LED color immediately
after switching-on and are stored as value Y(T.sub.bcal,
t.sub.0).
In a second step, the brightness Y and the board temperature
T.sub.b are measured for each LED color in the steady state and are
stored as value Y(T.sub.b, t.sub.1). Subsequently, the brightness
value Y(T.sub.b, t.sub.1) is converted to a board temperature
T.sub.b1 via the characteristic line Y=f(T.sub.b), wherein T.sub.b1
is the temperature for which the characteristic lines Y=f(T.sub.b)
have been normalized onto 1. The value Y(T.sub.b1, t.sub.1) is
stored as result.
In a third step, the correction factors are formed according to the
equation kYcal=Y(Tb1,t1)/Y(Tbcal,t0) which are only valid for the
board temperature T.sub.bcal measured during the calibration.
Optionally, a set of several calibration factors for different
board temperatures T.sub.bcal has to be generated during the
calibration.
FIG. 27 depicts a flow-chart for the brightness calibration of an
LED module which calibration serves for storing the brightnesses of
the LED colors in each individual LED module. The module
electronics of the LED module can read them from the memory and
compensate them. Thus, the colors of all LED modules of an LED
illuminating device (e.g. of a spotlight) illuminate similarly
bright if an external controller of the LED illuminating device
forces brightness set values for the different LED colors.
In a first step of the brightness calibration of the LED modules,
the brightness Y and the board temperature T.sub.b are measured for
each LED color immediately after switching-on the LED illuminating
device or the LED module and are stored as value Y(T.sub.bcal,
t.sub.0).
In a second step, a conversion to the brightness in the steady
state at a board temperature T.sub.b1 is converted for each color
according to Y(T.sub.b1)=Y(Tbcal,t0)*kYcal.
Thereby, the factor kY.sub.cal corresponds to the calibrating
correction factors determined according to the flow-chart according
to FIG. 26.
In a third step, the brightnesses of the LED colors converted to
the board temperature T.sub.b1 are stored in the respective LED
module.
The flow-chart depicted in FIG. 28 reflects the method for a color
calibration of the LED illuminating device or a spotlight. After
the start of the program, in a first step the measurement of the
spectrum is effected and resultantly derived of the brightness Y as
well as of the chromaticity coordinates x, y of each LED color of
the spotlight. Subsequently, the brightness of the spotlight is
converted to the board temperature T.sub.b1 by the characteristic
line Y=f(Tb) and the spectra are scaled to Y=Y(T.sub.b1).
In a second step, the calibration data x, y and Y(T.sub.b1) are
stored for each LED color in the spotlight. In a third step, the
calculation of the optimum luminous flux portions of the LED colors
from the measured spectra for N color temperature interpolation
points is effected by the precedingly described program-controlled
processing unit.
In a fourth step, the luminous flux portions of the LED colors for
N color temperature interpolation points are stored in the memory
of the spotlight and/or the luminous flux portions of the LED
colors are stored in table form dependent on the target
chromaticity coordinate, i.e. the chromaticity coordinates x,
y.
FIG. 29 shows a flow-chart of the color control of an LED
illuminating device designed as spotlight.
In the context of the color control of the LED illuminating device
a temperature-dependent power limiting is performed since the total
power of the LED illuminating device or the total current fed to
all LEDs of the LED colors must not exceed a specified, preferably
temperature-dependent threshold; because it does not make sense to
feed more current with increasing temperature and consequently
decreasing brightness of the LED illuminating device in the
expectation to therewith compensate the decrease in brightness of
single or several colors. The temperature further increases with an
increased feed of current and therewith of the total power of the
LED illuminating device so that the luminous efficacy further
decreases until single or several LEDs are overloaded and are
therewith destroyed or a hardware-based current limitation
intervenes.
A prerequisite for the color control of the LED illuminating device
depicted as flow-chart in FIG. 29 is the storage of calibration
data for N color temperature interpolation points and/or the
chromaticity coordinates table in the microprocessor of the LED
illuminating device or the LED modules with luminous flux portions
of the LED colors as function of the color temperature (CCT) and/or
of the chromaticity coordinates (x, y), the temperature
characteristic lines Y=f(T.sub.b) for each LED color and the
brightness and the chromaticity coordinates Y, x, y for each LED
color.
In a first step of the colorcontrol, the PWM factors PWM.sub.A of
the LED colors are determined for the desired chromaticity and the
brightness is determined optionally via interpolation. In a second
step, the board temperature T.sub.b is measured and, in a third
step, the temperature-dependent PWM correction factors are
determined for each color from the characteristic lines
fPWM=1/Y.sub.REL stored in the memory, wherein as value Y.sub.REL
the linear approximation function, quadratic approximation function
or third-grade approximation function according to the preceding
description is applied.
In a fourth step, it is checked if the total power P.sub.neu fed to
the LED illuminating device or the individual LED current I.sub.neu
exceeds a specified maximum value P.sub.max or I.sub.max. If this
is the case, a cut-off factor kCutoff is determined for limiting
the current or the power which factor is valid for all LED colors
and is determined according to kCutoff=P.sub.max/P.sub.neu or
kCutoff=I.sub.max/I.sub.neu.
If the new total power does not exceed the specified maximum value,
the factor is set to kCutoff=1.
In a fifth step, new PWM factors PWM.sub.T are determined according
to PWM.sub.T=PWM.sub.A*fPWM*kCutoff and the LEDs are controlled
with the new PWM factors PWM.sub.T, and subsequently one returns to
the first method step of the determination of the PWM factors for
the PWM.sub.A of the LED colors.
The basic brightnesses of the color channels measured in the
context of the calibration serve for the internal brightness
correction of the LED modules. Therewith, both the brightness
tolerances of the LED chips and the tolerances in the electronics
are calibrated. The color-dependent brightness correction factors
kY are then determined from these values in the context of the
calibration of the LED illuminating system and are stored. The
brightnesses determined during the calibration for each color are
converted to the working temperature T.sub.n via the temperature
characteristic lines which have been determined as being
representative in advance in the laboratory.
The internal basic brightnesses Y are read from all connected LED
modules in the context of the spotlight calibration, and the
brightness correction factors kY for all LED modules are calculated
and stored from the basic brightnesses with respect to the LED
module having the lowest brightness. They serve for the internal
brightness correction of the LED modules. The PWM commands received
from an external controller are multiplied with the brightness
correction factor kY internally in the LED modules so that all
connected LED modules represent the desired color with the same
brightness.
The brightness correction factors kY are calculated during the
calibration of the LED illuminating device for each channel as
follows: kY=Y.sub.min/Y wherein Y.sub.min denotes the minimum of
the basic brightnesses Y of all connected LED modules.
The parameters for the temperature characteristic lines are chosen
under application of a third-grade approximation function such that
the relative brightness for each color is normalized to 1 for the
working temperature T.sub.n and PWM=1. Thereby, the polynomial
coefficient a is 1. Since the temperature characteristic lines
depend on the peak current one has to revert to the respective set
of parameters in case of a peak current switch. All calibration
data related to the brightness is normalized to the working
temperature T.sub.n.
The maximum junction temperature of the LED chips indicates that
value for a cut-off temperature or a maximum board temperature
which is stored in the LED illumination and which must be below a
threshold for the maximum junction temperature of the LED
chips.
If the maximum board temperature T.sub.max is exceeded, the total
power of the LED module has to be uniformly reduced until the board
temperature T.sub.b is smaller or equal to T.sub.max. The power
reduction is effected via the color-independent power factor
k.sub.P.
The calculation of the dimming factors or PWM signals to be applied
module-internal is performed as follows. a) calculation of the
relative brightness Yrel dependent on the measured board
temperature Tb and of a curve Y=f(Tb) normalized to the value Y=1
at the board temperature Tn as well as of the PWM signal:
Y(Tb,PWM)=1+B*(Tb-Tn+dT)+C*(Tb-Tn+dT).sup.2+D*(Td-Tn+dT).sup.3
Y(Tn)=1+B*dT+C*dT.sup.2+D*dT.sup.3 with dT=E*(1-PWM.sub.intern)
being a power-dependent correction which typically is between -10
and -30.degree. C. Normalization of the power-corrected
characteristic line to 1 for the working temperature Tn:
Yrel=Y(Tb,PWM)/Y(Tn) b) Determining the temperature-dependent
correction factor kT (for each channel): kT=1/Yrel c) Determining
the power reduction k.sub.P for complying with or falling below the
maximum board temperature (for each module): If the maximum board
temperature Tmax is exceeded, the total power of the module has to
be uniformly reduced until Tb<=Tmax. The power reduction is
effected via the color-independent power factor k.sub.P. The time
constant t.sub.P (%/s) thereby describes the velocity of the power
regulation and m its slope. During the module start k.sub.P is 1.
If Tb>Tmax then the set power is reduced by the following
temperature-dependent factor: k.sub.P*=1-m(Tb-T.sub.max) (reduction
with the time constant t.sub.P) If Tb falls below T.sub.max, then
the power can be increased again: If k.sub.P<1, then
k.sub.p/=(1-m(Tb-T.sub.max)) (increase with time constant t.sub.P).
Alternatively, the spotlight can be turned off instead of being
dimmed if the limit temperature or shut-off temperature is
exceeded, if no brightness modification during the operation is
allowed. In this case k.sub.p is 0, if Tb>T.sub.max The power
factor k.sub.P is maximum k.sub.P=1. d) Determination of the
dimming factors or PWM signals per channel theoretical necessary
due to temperature: PWM.sub.theo=PWM.sub.soll*kT*kY
PWM.sub.theo,max=maximum of PWM portions PWM.sub.theo determined
for all colors e) Determining the possible relative brightness of
the module Yrel per LED module: If PWM.sub.theo,max<=1, then:
Yrel module=k.sub.P If PWM.sub.theo,max.gtoreq.1, then: Yrel
module=k.sub.P/PWM.sub.theo,max f) Data for a group matching: All
connected LED modules receive the command SetGroupBrightness from a
central power control unit, through which the relative brightness
of the temperature-related darkest LED module in the spotlight is
communicated to them. All other LED modules adjust their brightness
to this brightness to avoid temperature-related brightness
gradients. Each LED module sends its possible relative brightness
Y.sub.rel,module to the central power control unit for the group
matching which central power control unit determines the brightness
of the (temperature-related) darkest LED module and sends this as
Y.sub.rel,Group to all LED modules in order that these can adapt
(reduce) their brightness to it: Y.sub.rel,Group=minimum of the
values Y.sub.rel,module received from all LED modules. g) Group
matching LED modules Each LED module aligns its brightness to the
group brightness. The factor k.sub.Group for the group matching is
calculated as follows; the default value for k.sub.Group is 1
k.sub.Group=Y.sub.rel,Group/Y.sub.rel,module h) Calculation of the
internal dimming factors or PWM signals
.function. ##EQU00005## Subsequently, all LED modules of the same
color illuminate with identical brightness.
It is necessary for the power stabilization within a spotlight to
normalize the calculated relative luminous flux portions per
primary color. If the spotlight, e.g., is controlled such that the
PWM signals are normalized to the maximum value PWMmax=1, then the
maximum possible brightness is achieved in each case. However, this
does not make sense since on the one hand the brightness of an
adjusted color should be constant over the operation temperature
what can be compensated very simply with the aid of the
temperature-brightness characteristic lines. On the other hand, the
LED power generated therewith can, however, be too high depending
on the cooling of the spotlight so that the LED spotlight would
reach already shortly its uppermost threshold temperature (shut-off
temperature) and would turn off. In case of passive cooling, the
spotlight generally must be operated with an internal dimming
factor to become not too hot. This internal dimming factor depends
very strongly on the mixing ratio of the LED colors and therewith
on the color temperature or the chromaticity coordinates.
The relative luminous flux ratio calculated for any color or for a
color mode is therefore related to a maximum LED power P.sub.max(W)
which is stored in the memory of the spotlight.
To be able to calculate the actual power of an adjusted color
mixture and to normalize it onto P.sub.max, the powers P.sub.i(W) @
PWM=1 are stored during the calibration in the spotlight for each
color channel.
Compensation of the Temperature-Related Color Shift at LED
Modules
A variation of the color temperature dependent on the temperature
can be observed in case of spotlights constructed from LED modules.
The extent amounts to ca. 300 K for the settings 3200 K and 5600 K.
This effect can be traced back to the temperature-related shift of
the dominant wavelength, in particular of the red and yellow LEDs.
Since a calibration is effected by a measurement of the spectra and
calculation of the necessary luminous flux portions in the warm
state, the spotlight, however, has a lower temperature during the
warming up or in the dimmed state, a spectral shift effects an
increase of the color temperature.
The temperature compensation implemented in the LED modules
according to the precedingly described methods compensates only the
brightnesses and takes care that the relative luminous flux
portions of the color mixture remain constant over the temperature.
The spectra depicted in FIGS. 30 and 31 clarify the differences
between the cold and warm spectra for the settings 3200 K (FIG. 30)
and 5600 K (FIG. 31), which have been measured at NTC (Negative
Temperature Coefficient Thermistor) temperatures of 70.degree. C.
and 25.degree. C. and which occur with the method of constant
luminous flux portions implemented hitherto. The
temperature-related color shift does hereby not exactly run along
the Planckian locus, in particular at lower color temperatures
deviations of up to 5 threshold units from the Planckian locus
occur. Due to this fact, not only the CCT deviation but also the
deviation of the chromaticity coordinates (dx, dy) is compensated
according to the invention.
FIG. 32 shows the CCT deviation cold-warm dependent on the color
temperature,
FIG. 33 shows the deviation of the chromaticity coordinates dx, dy
(cold-warm) dependent on the target chromaticity coordinate x for
target chromaticity coordinates x, y along the Planckian locus in
the color temperature range between 2200 K and 24000 K and FIG. 34
shows the optimum luminous flux portions warm and cold as function
of the color temperature CCT.
On the spotlight level, the following methods are possible for the
compensation of the color shift: a) Entering a compensation
algorithm for the color temperature correction .DELTA. CCT=f(CCT,
T.sub.NTC) in connection with calibration data for an NTC
temperature. This compensation method can be easily performed but
is comparably imprecise since deviations from the Planckian locus
are not compensated and is only applicable for color temperature
adjustments but not for any chromaticity coordinates, e.g., not for
effect colors. The compensation algorithm for the color temperature
correction can be determined experimentally or mathematically. In
case of an experimental determination, the optimum luminous flux
portions for different CCT interpolation points in the warm
operation state (T.sub.NTC warm) as well as the
brightness-temperature characteristic lines are determined for a
spotlight, and the spotlight is adjusted in the cold state
(T.sub.NTC cold) to different set color temperatures. Subsequently,
the color temperature of the emitted light is measured and the
difference between the target color temperature and the measured
color temperature is plotted dependent on the target color
temperature. An approximation function, e.g., a polynomial is
determined for these pairs of values. In case of a mathematical
determination of a compensation algorithm for the color temperature
correction, it is assumed that the optimum luminous flux portions
for different CCT interpolation points in the warm operation state
(T.sub.NTC warm) of a spotlight are present. Then, the spectra of
the single colors are measured in the cold operation state
(T.sub.NTC cold) and these "cold spectra" are mixed for different
CCT interpolation points by means of the luminous flux portions
determined for the warm operation state T.sub.NTC warm and the
color temperature is calculated from the mixed spectrum obtained in
this way. The difference between the target color temperature and
the color temperature calculated from the cold spectra is plotted
dependent on the target color temperature. An approximation
function (e.g. a polynomial) is determined for these pairs of
values. The approximation function obtained in this way represents
the color temperature correction .DELTA. CCT.sub.cold to be applied
dependent on the target color temperature for a cold spotlight.
Typically, the NTC temperature lies in operation between T.sub.NTC
warm and T.sub.NTC cold. The color temperature correction .DELTA.
CCT.sub.cold (CCT.sub.target) determined dependent on the target
color temperature is linearly interpolated according to the actual
T.sub.NTC value:
.DELTA.CCT(CCT.sub.target,T.sub.NTC)=.DELTA.CCT.sub.cold(CCT.sub.target)/-
(T.sub.NTC warm-T.sub.NTC cold)*(T.sub.NTC-T.sub.NTC cold) The
software then provides the spotlight the color temperature
corrected for the value .DELTA. CCT(CCT.sub.target, T.sub.NTC)
instead of the desired target color temperature. The method of the
color temperature correction leads to correct highly correlated
color temperatures of the emitted light at different NTC
temperatures. It does, however, not have the ability to compensate
optionally additional occurring color deviations from the Planckian
locus since the color deviation to be compensated rarely accidental
runs exactly along the Planckian locus due to the
temperature-conditional shift of the dominant wavelength.
Alternatively, the optimum luminous flux portions can also be
determined for the cold operation state and the correction function
can be determined by means of the spectra or the measurement data
of the spotlight in the warm operation state. b) Entering a
correction algorithm for the correction of the chromaticity
coordinates .DELTA.x and .DELTA.y=f(x.sub.target, T.sub.NTC) or
.DELTA.x and .DELTA.y=f(CCT.sub.target, T.sub.NTC) and the
calibration data for an NTC temperature. This compensation method
also can be simply performed, however, it works for the correction
of the chromaticity coordinates, e.g., for a maximum brightness.
However, it does not provide optimum luminous flux portions and
holds the danger of a CRI deterioration. Additionally, it is only
applicable for a color temperature adjustment, but not for any
chromaticity coordinates, e.g., for effect colors. This
compensation method requires two correction functions for the
chromaticity coordinates x and y. The correction functions for the
correction for the chromaticity coordinates can be determined,
analogously to the compensation algorithm for the color
temperature, either experimentally or mathematically. The
corrections of the chromaticity coordinates .DELTA.x,
.DELTA.y.sub.cold (CCT.sub.target) determined dependently on the
target chromaticity coordinates are linearly interpolated according
to the actual T.sub.NTC value:
.DELTA.x,.DELTA.y(CCT.sub.target,T.sub.NTC)=.DELTA.x,.DELTA.y.sub.cold(CC-
T.sub.target)/(T.sub.NTC warm-T.sub.NTC cold)*(T.sub.NTC-T.sub.NTC
cold) The software then provides the spotlight the chromaticity
coordinates corrected for the values .DELTA.x(CCT.sub.target,
T.sub.NTC) and .DELTA.x(CCT.sub.target, T.sub.NTC) instead of the
chromaticity coordinates of the desired target color temperature.
Also here, the optimum luminous flux portions for the cold
operation state can be alternatively determined, and the correction
functions can be determined by means of the spectra or the
measurement data of the spotlight in the warm operation state. The
described method of the correction of the chromaticity coordinates
leads to correct chromaticity coordinates along the Planckian locus
of the emitted light at different NTC temperatures. Desired color
temperatures can therewith be adjusted exactly along the Planckian
locus. Since in case of this compensation of chromaticity
coordinates some colors have to be mixed to the stored optimum
luminous flux ratio and there are, in case of three channels,
partially theoretical unlimited possibilities of combination, the
admixing of colors is possibly effected unfavorably with respect to
an optimum color reproduction and mixed-light capability with film.
This uncertainty is solved with the compensation method described
hereinafter under c). c) Interpolation optimum mixture=f(CCT,
T.sub.NTC) and chromaticity coordinates=f(T.sub.NTC) and
determining the calibration data (optimum mixture and chromaticity
coordinates) for two NTC temperatures. These compensation methods
results in the best color rendering index (CRI), represents the
most precise (x, y) method for the mixtures optimized towards the
color reproduction and the brightness, represents the most precise
(x, y) method for mixtures and is applicable for any chromaticity
coordinates. However, it requires a higher effort for the software
development (calibration, spotlight, colorimetry). The time effort
during the spotlight calibration is increased only marginally.
Without application of this compensation method, the spotlight
would be only calibrated in the warm and therewith typical
operation state, wherein the time effort for the calibration is
essentially composed of inserting the spotlight into the
measurement apparatus, connecting the spotlight to the supply and
control devices as well as starting the calibration software and
the heating-up period to the calibration temperature T.sub.NTC
warm. The actual detection of the spectra is effected in a matter
of seconds. During the compensation method c) "cold spectra" are
co-detected only prior to the start of the heating-up phase and are
accordingly processed by the software, what can be effected within
a few seconds and does not require additional activities of the
user. This method can be applied for the following modes: a.
Adjusting a desired color temperature with best possible color
reproduction and mixed-light capability, i.e. color-rendering
optimized. During calibration, the spectra of the primary colors
are detected in the cold (T.sub.NTC cold) as well as in the warm
(T.sub.NTC warm) state and optimum luminous flux portions of the
used LED colors are calculated for some CCT interpolation points
and are stored in the spotlight or the control device:
Y.sub.rel-warm (CCT) optimal luminous flux portions dependent on
the CCT for T.sub.NTC warm Y.sub.rel-cold (CCT) optimal luminous
flux portions dependent on the CCT for T.sub.NTC cold These optimum
luminous flux portions lead both in the cold and in the warm state
to color-rendering optimized light mixtures which match exactly the
chromaticity coordinates of the desired color temperature. For NTC
temperatures unequal T.sub.NTC warm or T.sub.NTC cold the optimum
mixture can be obtained by interpolation:
Y.sub.rel(CCT,T.sub.NTC)=Y.sub.rel.sub.--.sub.cold(CCT)+(T.sub.NTC-T.sub.-
NTC
cold)*(Y.sub.rel.sub.--.sub.warm(CCT)-Y.sub.rel.sub.--.sub.cold(CCT))/-
(T.sub.NTC warm-T.sub.NTC cold) If a color temperature is to be
adjusted which lies between two CCT interpolation points then the
mixtures of both CCT interpolation points are calculated for the
actual NTC temperature as precedingly described and are
subsequently interpolated between the two CCT interpolation points
such that the desired target color temperature is achieved. b.
Setting of any chromaticity coordinates or effect colors with best
possible luminous efficacy or brightness, i.e.
brightness-optimized. For the calculation of any
brightness-optimized chromaticity coordinates which can be both
"white" colors having any color temperature and any effect colors
which lie within the depictable LED gamut, only the tristiumulus
values X, Y, Z of the used primary colors are required according to
the laws of additive color mixture. The tristimulus values X, Y, Z
can be calculated from the chromaticity coordinates x, y and the
brightness-proportional value Y with the aid of the generally known
formula of colorimetry so that it is sufficient to know the values
x, y and Y dependent on the NTC temperature. During application of
the brightness-temperature characteristic lines one can assume that
the tristimulus value Y remains constant. Thus, it is sufficient to
only store the values x, y dependent on the NTC temperature. For
this purpose, chromaticity coordinates of the LED primary colors
are calculated from their "cold spectra" and their "warm spectra"
during the calibration and are stored together with the brightness
value Y in the memory of this spotlight or of the control device:
The chromaticity values of the primary colors needed for the
calculation of the mixtures for adjusting any colors with maximum
brightness can be calculated by linear interpolation dependent on
the actual NTC temperature:
x(T.sub.NTC)=x.sub.cold+(T.sub.NTC-T.sub.NTC
cold)*(x.sub.warm-x.sub.cold)
y(T.sub.NTC)=y.sub.cold+(T.sub.NTC-T.sub.NTC
cold)*(y.sub.warm-Y.sub.cold) Y(T.sub.NTC)=Y.sub.warm according to
the applied temperature-brightness characteristic lines
FIG. 35 shows a graphic of the measured color temperature of the
5-channel LED module dependent on the NTC temperature for the
setting CCT=3200 K with implemented correction of the spectral
shift according to method c) and FIG. 36 shows a graphic of the
measured color temperature of an LED module dependent on the NTC
temperature for the setting CCT=5600 K with implemented correction
of the spectral shift according to method c) in comparison to the
behavior without correction of the spectral shift with only acting
of the temperature compensation.
As precedingly explicated, for each LED primary color the
characteristic lines Y.sub.rel=f(T.sub.NTC, PWM.sub.i) are
implemented:
Y(.sub.T.sub.--.sub.NTC)=A+B*(T.sub.NTC-Tn+dT)+C*(T.sub.NTC-Tn+dT).sup.2+-
D*(T.sub.NTC-Tn+dT).sup.3 (formula 9) with dT=E*PWM (formula 10)
wherein Y(.sub.T.sub.--.sub.NTC) brightness dependent on the NTC
temperature A, B, C, D polynomial coefficients of the
characteristic lines T.sub.NTC actual NTC temperature Tn working
temperature If the curves are normalized to
Y(.sub.T.sub.--.sub.NTC)=1 @ T.sub.NTC=Tn, then the polynomial
coefficient A=1. dT correction value dependent on the actual LED
power E "power parameter" PWM LED PWM control signals The micro
controller calculates for each color the temperature correction
factor kT=1/Y(.sub.T.sub.--.sub.NTC) during the spotlight operation
dependent on the actual NTC temperature. The PWM signals calculated
for each adjustment of a desired color are multiplied with the
correction factor kT calculated for each color. Thereby, the
brightness of the color is kept constant over the operation
temperature. Thereby, the following effects are accounted for:
Temperature dependency of the brightness per color with
power-dependent temperature correction of the characteristic lines
("power parameter E" in connection with the internal PWM) The
curves are described by a third-grade polynomial, coefficients of
the temperature characteristic line: A, B, C, D as well as power
parameter E.
Since the LED power of same-color LEDs can vary at the same dimming
factor (PWM) at the same current due to forward voltage tolerances,
because the temperature difference between the value measured at
the NTC and the junction of the LED depends on the forward voltage,
a correction is performed for which the power-dependent temperature
correction is individually calculated for each LED module dependent
on the individual LED forward voltages UF.
It follows from the generally known formula for the thermal
resistance Rth=dT/dP that the temperature difference between NTC
and junction is directly proportional to the transmitted power. The
LED power in turn is directly proportional to the forward voltage:
P=UF*I.
From this it follows that the temperature difference between the
NTC and the junction dT is directly proportional to the forward
voltage of the LEDs: dT UF.
The power parameter E empirically determined for a typical LED
module is thus directly proportional to the forward voltage UF of
the LEDs. If the forward voltage of the individual LEDs deviates
from that LED for which the characteristic lines have been
determined, then formula 9 can be extended as follows:
dT=E*U.sub.F/U.sub.measured*PWM (formula 9a) Thereby, UF is the
forward voltage of the LED color of the individual LED module
U.sub.measured is the forward voltage of the LED color of the LED
module at which the typical brightness-temperature characteristic
lines have been recorded.
The individual forward voltage UF additionally depends to a low
extent on the temperature. It can either approximately be regarded
as constant and can be determined once, e.g., during the
calibration and be stored or it is in a more precise method
measured by the micro controller during the spotlight operation or
the value determined during the calibration is corrected dependent
on the actual NTC temperature. In the data sheets of the LED
manufactures the according data dUF/dT can be found.
For determining the temperature characteristic lines dependent on
the dimming factor (PWM) and the forward voltages the following
method steps are thus provided which are schematically depicted in
the flow-chart according to FIG. 37, wherein all graphics to be
evaluated have to be normalized to Y=1 at working temperature
T.sub.NTC=Tn. 1. Performing the measurements (with spectrometer)
Y.sub.PWM100=f(T.sub.NTC)brightness=f(temperature) for PWM=100%
Y.sub.PWM20=f(T.sub.NTC)brightness=f(temperature) for PWM=20%
U.sub.measured forward voltage at 25.degree. C. 2. Normalization of
the measured characteristic lines to Y=1 at T.sub.NTC=T.sub.n (e.g.
75.degree. C.) 3. Mathematical determination of the temporally
polynomial coefficient B.sub.temp, C.sub.temp, D.sub.temp for
measured curve PWM=100 from 4 interpolation points for a
third-degree polynomial having the form
Y.sub.PWM100=A+B*(T.sub.NTC-Tn)+C*(T.sub.NTC-Tn).sup.2+D*(T.sub.NTC-Tn).s-
up.3 The coefficient A is thereby 1 due to the preceding
normalization to Y=1 at T.sub.NTC=T.sub.n 4. Experimental
determination of dT.sub.PWM20 for the fitted curve PWM=20
Y(.sub.T.sub.--.sub.NTC)=1+B.sub.temp*(T.sub.NTC-Tn+dT)+C.sub.temp*(T.sub-
.NTC-Tn+dT).sup.2+D.sub.temp*(T.sub.NTC-Tn+dT).sup.3 (parameter dT
is thereby varied until this formula results in an optimum
approximation to the measured curve PWM=20.) 5. Extrapolation of
dT.sub.PWM20 to dT.sub.PWM0:dT.sub.PWM0=5/4*dT.sub.PWM20 6.
Determination of polynomial coefficients B.sub.1, C.sub.1, D.sub.1
for the precedingly extrapolated curve with PWM=0 4 interpolation
points from following curve:
Y(.sub.T.sub.--.sub.NTC)=1+B.sub.temp*(T.sub.NTC-Tn+dT.sub.PWM0)+C.sub.te-
mp*(T.sub.NTC-Tn+dT.sub.PWM0).sup.2+D.sub.temp*(T.sub.NTC-Tn+dT.sub.PWM0).-
sup.3 result in a new equation for PWM=0
Y(.sub.T.sub.--.sub.NTC)=1+B.sub.1*(T.sub.NTC-Tn)+C.sub.1*(T.sub.NTC-Tn).-
sup.2+D.sub.1*(T.sub.NTC-Tn).sup.3 7. Experimental determination of
dT.sub.PWM100 for the measured curve PWM=100 (with polynomial
coefficients B.sub.1, C.sub.1, D.sub.1)
Y(.sub.T.sub.--.sub.NTC)=1+B.sub.1*(T.sub.NTC-Tn+dT.sub.PWM100)+C.sub.1*(-
T.sub.NTC-Tn+dT.sub.PWM100).sup.2+D.sub.1*(T.sub.NTC-Tn+dT.sub.PWM100).sup-
.r (parameter dT to be varied until optimal approximation to the
measured curve PWM=100) 8. Determination of the temporally power
parameter E.sub.temp Approach:
dT.sub.PWM100=E.sub.temp*PWM.fwdarw.E.sub.temp=dT.sub.PWM100/PWM 9.
Determination of the general power parameter E.sub.1 Approach:
.function. ##EQU00006## From this it follows:
E.sub.1=E.sub.temp/U.sub.measured If the individual forward voltage
is not to be considered, then E.sub.1=Etemp 10. The general
temperature characteristic lines dependent on the PWM as well as on
the forward voltage now read:
Y(.sub.T.sub.--.sub.NTC)=1+B.sub.1*(T.sub.NTC-Tn+dT)+C.sub.1*(T.sub.NTC-T-
n+dT).sup.2+D.sub.1*(T.sub.NTC-Tn+dT).sup.3 with
dT=E.sub.1*PWM*U.sub.F
If one looks at the brightness-temperature characteristic lines for
the colors yellow . . . orange . . . red then one realizes that the
curves for yellow (ca. 590 nm) run most steeply, for orange to red
(ca. 620 nm) increasingly more flat. The brightness modification
between Y(20.degree. C.)/Y(74.degree. C.) measured at an LED module
with yellow (dominant wavelength 592 nm) and red (dominant
wavelength 620 nm) has the factor 1.80 for the red or 3.19 for the
yellow LEDs. Only 28 nm difference in the dominant wavelength lie
in between. From this it is obvious that already typical tolerances
of the dominant wavelength of few nanometers have a strong effect
on to the actual brightness temperature characteristic lines.
Due to this fact, a correction or adaptation of the stored
temperature coefficients dependent on the dominant wavelength, in
particular for AlInGaP chips (amber, red) is performed according to
the invention, wherein the characteristic lines are individually
adapted for each LED module onto the individual dominant
wavelengths.
The correction of the brightness-temperature characteristic lines
for this effect can be effected according to the following
principle: Several brightness-temperature characteristic lines per
color are recorded in the laboratory at LED modules of different
dominant wavelengths From this, the polynomial parameters A . . . E
are determined for each color dependent on the dominant wavelength.
In the context of the LED module calibration, the spectra of the
LED colors as well as the according NTC temperature are detected
for each LED module. This can be effected in the context of the
module calibration and module selection and does generally not
represent any additional effort. The dominant wavelengths per color
are calculated from this spectrum. The polynomial parameters A . .
. E determined in advance at single modules are corrected according
to the deviation of the individual dominant wavelength of the
module to be calibrated from the dominant wavelength of the module
from which the characteristic lines have been determined. The
conversion of the polynomial parameters to an LED having certain
dominant wavelengths can be effected by a linear interpolation of
the polynomial parameters of two known curves of two LEDs having
different dominant wavelengths to the new dominant wavelength. The
most precise results are obtained if the dominant wavelengths of
the original curves as well as the dominant wavelength onto which
it should be converted lie together as close as possible. Thereby,
it must not be interpolated between given curves of different LED
technologies like AlInGaP and InGaN. If one, e.g., requires the
curve for a third-degree polynomial together with polynomial
parameters A . . . D for a yellow LED having the dominant
wavelength I_dom_yellow1, then one requires additionally the curve
together with the polynomial parameters A . . . D for a similar LED
having a different dominant wavelength I_dom_yellow2 (with a
somewhat higher uncertainty also orange or red). The polynomial
parameters A . . . D for a yellow LED having a dominant wavelength
I_dom_yellow3 are then obtained by a linear interpolation of the
polynomial parameters for the curves with I_dom_yellow1 or
I_dom_yellow2 dependent on the wavelength difference. The general
procedure is shown in FIG. 38 by means of the original curves for a
yellow and a red LED as well as the curves derived from it for two
theoretic yellow LEDs, the dominant wavelengths of which deviate by
+/-3 nm from the original yellow curve. An advantage of this method
is that, during spotlight operation, the brightness of each LED
module can be kept constant according to its individual valid
temperature-brightness characteristic line without the necessity
that these have to be individually and metrologically determined in
time consuming measurements of the brightness over the temperature.
Instead of that, it is sufficient for determining the individual
temperature-brightness characteristic line to know this curve for a
"typical" LED module and to further detect the spectra of the
individual LED modules in the cold state, what is possible with an
extremely low time effort and would typically be effected in the
context of the calibration anyway. Naturally, this method can be
applied for all LED colors. However, the strongest effect will
occur for the AlInGaP colors yellow . . . orange . . . red.
Stabilization of Luminous Efficacy
Since the luminous efficacy of the mixtures and therewith the
brightness vary due to the temperature-dependent tracking of the
color-reproduction optimized mixtures and additionally the
individually stored optimum luminous flux portions of the
color-reproduction optimized mixtures can let occur mixtures having
different luminous efficacies and therewith different brightnesses
at different spotlights, two methods for the color stabilization
and brightness stabilization are applied to extend the brightness
stabilization and to adapt several spotlights to a
color-reproduction optimized white mode via the luminous efficacy:
normalization of the luminous efficacy dependent on the board
temperature set match of luminous efficacy between different
spotlights
Firstly, on the one hand the brightness-temperature characteristic
lines dependent on the pulse-width modulation have been applied for
the color stabilization and brightness stabilization and the
luminous flux portions of a color mixture for different NTC
temperatures calculated for the warm operation state have been kept
constant.
On the other hand, a "power normalization" has been introduced to
keep the maximum LED power for each color mixture constant when the
warm operation state has been reached. Therewith, a premature
reaching or exceeding of a switch-off temperature is avoided. An
individual "internal" power dimming factor is calculated and
applied for each adjusted color mixture with the aid of the power
normalization (e.g., 5 W LED power per module). Therewith, each
color mixture can be adjusted with optimum brightness or optimum
internal dimming factor without reaching or exceeding the shut-off
temperature at normal ambient conditions. Thereby, the power
normalization is effected selectively for the warm operation state
because here a higher LED current or a higher LED power has to be
applied due to the negative brightness-temperature characteristic
of the LEDs to keep the brightness of the spotlight constant over
the temperature. At temperatures below the switch-off temperature
the spotlight is automatically operated at a lower power. To keep
the brightness constant without thereby ever having to adjust a
higher power than Pmax, this maximum power must be reached only at
the switch-off temperature.
Each selected chromaticity coordinate could be set in each case
with the highest possible brightness being also constant over the
operation temperature by both preceding methods. The measured
brightness variations per selected chromaticity coordinates varied
by less than 1% between cold and warm.
It is disadvantageous that the adjusted chromaticity coordinates
changed over the operation temperature due to the spectral shift of
the used LED primary colors. The extent of the chromaticity
coordinate variation depended on the chromaticity coordinate as
well as on the respective color mixture and amounted to the
dimension of 300 K between cold and warm, wherein the color
temperature decreased with increasing temperatures since the effect
of the temperature-dependent spectral shift is pronounced in
particularly for the AlInGaP LEDs in the yellow to red color range.
The variation of the dominant wavelength amounts to ca. 0.1 nm/K
for yellow, orange and red AlInGaP LEDs. A remedy was effected via
the precedingly described compensation of the temperature-dependent
spectral shift by essentially duplicating the calibration data for
the warm to the cold state and a temperature-dependent linear
interpolation. This algorithm could seriously ameliorate the
constancy of the chromaticity coordinates over the operation
temperature.
However, despite power normalization and application of the
brightness-temperature characteristic lines partly massive luminous
flux variations of an adjusted color of up to much more than 10%
between the cold and warm operation state occurred by the
compensation of the spectral shift. Extent as well as direction of
the brightness variation depend on the chosen chromaticity
coordinate or the color mixture and could thus not be determined or
compensated without further ado.
The reason for these brightness variations at constant chromaticity
coordinates is that the luminous efficacy of the respective
mixtures varies with the operation temperature due to the
temperature-dependent tracking of the luminous flux portions or the
modification of the importance of the single LED primary colors.
This effect is completely independent on the brightness-temperature
behavior of the LEDs. The normalization of these mixtures varying
with the temperature to a constant LED total power used hitherto
led inevitably to non-constant brightnesses due to the varying
luminous efficacies of the LED mixtures.
This problem is solved by an extended brightness stabilization via
the luminous efficacy as follows:
For all optimum luminous flux portions of the CCT interpolation
points stored in the memory the according luminous efficacies for
the warm operation state .eta..sub.NTC.sub.--.sub.warm(CCT,
T.sub.NTC.sub.--.sub.warm) are additionally calculated and stored
in the memory. During the operation, the actual luminous efficacy
.eta..sub.NTC(CCT, T.sub.NTC) is calculated from the mixtures
tracked for deviating operating temperatures. The luminous efficacy
correction factor
k.eta.=.eta..sub.NTC.sub.--.sub.warm/.eta..sub.NTC is calculated
from the ratio of those two values and the set PWM portions of the
LED mixture are multiplied with this factor. By this method, both
the chromaticity coordinates and the brightness remain constant
over the operation temperature.
Set Match of Luminous Efficacy
Due to the module-internal temperature compensation and the
calibration data Y, x, y (per color) stored in the spotlight, each
spotlight makes only sure that the adjusted color (CCT or x, y) is
correct. In a set consisting of several spotlights all spotlights
have then the same color--but possibly different brightnesses.
Even in case of good selection of the LED chips both the
chromaticity coordinates and the luminous efficacies of the used
LED primary colors can vary from spotlight to spotlight since the
optimum luminous flux portions for the cold and the warm operation
state are determined and stored for each spotlight for different
CCT interpolation points to adjust color-reproduction optimized
color temperatures. These optimum luminous flux portions and
according luminous efficacies can vary due to LED tolerances from
spotlight to spotlight. Thus, different spotlights require
individual LED mixtures to safely adjust the desired color.
If now a set consisting of several spotlights would be adjusted
together onto a certain color temperature and the color mixture of
each spotlight would be related to the same maximum total power
P.sub.max,warm, then the luminous efficacies of the single
spotlights could deviate by more than 30% from each other for the
same color temperature. Analogously, the brightness of the
spotlights would vary correspondingly--at the same color
temperature adjustment and LED power. It would be impossible to
adjust a set of spotlights to the same color at the same
brightness.
To make sure that all spotlights connected to a controller have the
same brightness, a brightness matching function, e.g., by the
controller, is necessary by which the respective brighter
spotlights are adjusted, i.e. reduced, for each color to the lowest
brightness within the set.
This problem is solved by a "luminous efficacy set match" as
follows:
The luminous efficacy in the warm state is additionally calculated
and stored for the color mixtures of all CCT interpolation points
for the color-reproduction optimized white mode. For all
spotlights, which are connected together to a set, the smallest
luminous efficacy per CCT interpolation point is determined of all
spotlights belonging to the set and is stored as set luminous
efficacies of the CCT interpolation points in all spotlights. From
this, the set luminous efficacy correction factor is determined
dependent on the CCT and the actual NTC temperature during the
operation:
k.eta.Set(CCT,T.sub.NTC)=.eta.Set(CCT,T.sub.NTCwarm)/.eta.(CCT,T.sub.NTC)
and the determined PWM portions are multiplied therewith, i.e., all
spotlights are adjusted per CCT interpolation point to the
brightness of the lowest luminous efficacy within the set.
Therewith, all spotlights of a set illuminate in the
color-reproduction optimized white mode with the same brightness
which does not vary anymore over the temperature. Likewise, the
chromaticity coordinates remain constant over the whole operation
temperature due to the precedingly described compensation of the
spectral shift.
This method establishes two options: a) Generation of any CCTs with
maximum possible brightness. The brightness of an adjusted CCT is
constant both within all spotlights of a set and over the
temperature. However, the brightness might vary according to the
corresponding set luminous efficacy due to a variation of the CCT.
b) Generation of any CCTs with constant brightness so that the
brightness of all selectable CCTs is constant both within all
spotlights of a set and over the temperature. Upon variation of the
CCT the brightness remains constant. Therefore, only the minimum
value of the set luminous efficacies .eta.Set(CCT, T.sub.NTC warm)
is determined over all CCTs, .eta.Set.sub.min(T.sub.NTC warm) and
the actual set luminous efficacy correction factor k.eta.Set(CCT,
T.sub.NTC)=.eta.Set.sub.min/.eta.(CCT, T.sub.NTC) is applied. In
this manner, all spotlights within a set can generate any color
temperatures with identical brightness.
For performing this method the following data is necessary:
Y.sub.rel cold=f(CCT) optimized luminous flux portions for CCT
interpolation points, cold operation state Y.sub.rel warm=f(CCT)
optimized luminous flux portions for CCT interpolation points, warm
operation state P100.sub.i powers per LED primary color @ PWM=1
Y100.sub.i brightness per LED primary color for warm operation
state @ PWM=1 T.sub.NTCwarm NTC temperature for warm operation
state T.sub.NTCcold NTC temperature for cold operation state
.eta.Set=f(CCT) set luminous efficacy for warm operation state
The following formula serves for the calculation of the luminous
efficacy .eta. of a color mixture:
Given are: Y.sub.rel,i=f(CCT,T.sub.NTC): luminous flux portions for
desired CCT for actual NTC temperature PWMi=Y.sub.reli/Y100.sub.i
PWM signals for adjusting the luminous flux portions Total
brightness=.SIGMA.PWMi*Y100, total brightness of the actual mixture
before correction Total power=.SIGMA.PWMi*P100.sub.i total power of
the actual mixture before correction .eta.=total brightness/total
power luminous efficacy of the actual mixture (formula 11)
The set match can, e.g., be effected within the calibration. All
spotlights of a manufacturing series can also be considered as set:
Then additionally all sets of a manufacturing series would
represent the desired CCTs having the same brightness.
The set match can be carried out by the controller in case of a
composition of individual sets. Therefore, it reads in the
according spotlight calibration data, determines the minimum set
luminous efficacies and stores these as set calibration data in the
calibration data.
The set match is done as follows: The controller reads in from all
connected spotlights: Y.sub.rel warm=f(CCT) optimized luminous flux
portions for CCT interpolation points, warm operation state
P100.sub.i powers per LED primary color @ PWM=1 Y100.sub.i
brightness per LED primary color for warm operation state @ PWM=1
The controller calculates the luminous efficacies of the CCT
interpolation points for T.sub.NTC warm:.eta..sub.warm,k=f(CCT) for
all connected spotlights and for all CCT interpolation points
according to formula 1 The controller determines the minimum
luminous efficacy of the spotlight set to .eta.Set=f(COT) from all
spotlights per CCT interpolation point from the values
.eta..sub.warm,k=f(CCT) The controller writes into the EEPROM of
the spotlights the set luminous efficacies .eta.Set=f(CCT)
(therewith, the set match is effected.) If a color temperature is
adjusted at the spotlight, then the colorimetric functions
calculate the actual luminous efficacy .eta.(CCT, T.sub.NTC) for
each actual color mixture dependent on the NTC temperature and
determined from it the actual set luminous efficacy correction
factor
k.eta.Set(CCT,T.sub.NTC)=.eta.Set.sub.min/.eta.(CCT,T.sub.NTC). For
the PWM controlling, the determined PWM signals are multiplied with
the set luminous efficacy correction factor k.eta.Set(CCT,
T.sub.NTC).
With the indices i for the color and k for the spotlights
To ameliorate the correct color as well as the color fidelity
during dimming, non-perfectly linear dimming characteristic lines
are recorded per color channel by determining approximation
functions for the dimming characteristic lines per color, storing
dimming coefficients a and x per color in the spotlight and
correcting the PWM control signals according to the characteristic
line.
* * * * *